Carry on size for flight originating on United transferring to Lufthansa booked on one ticket?

That United agent has never flown Lufthansa methinks.
They are way more fussy in Europe.
I’ve experienced Lufthansa being particular!
Go with measurements that are the smallest of all the airlines you are flying with.

Hi ladybug. I've flown several times on a similar itinerary; i.e. United flight connecting with a flight with another airline. Note that there are at least 5 separate topics here that are easy to think of as 1, but are really separate.

1. whether you can bring your bag aboard a given flight as carryon
2. whether the bag needs to be checked at ticket counter.
3. whether bag can / needs to be checked at gate
4. if not carryon, whether you will pick up at plane as you disembark, or in terminal at baggage claim
5. what fee, if any, you will pay for your bag.
...whew!
5 is the easy part - baggage fees. Look at your receipt / ticket info from United. It will state the charges for baggage. These rules about bag charges apply to all legs of your flight, regardless of airline. All of the other items 1-4, like you said, are somewhat unpredictable / at the whim of the ticket agent / gate agent. My advice is to not try to predict / overanalyze. Just make your best guess as to what will happen, but roll with whatever happens. In general, in my opinion, it's overkill to measure / buy your bag down to the inch, because so much of it is out of your control anyway. Have a great trip!

You haven't even discussed weight. Is it 8kg ( 17+ lbs.) In my experience, you also have the chance of your bag being weighed. I think the weight is harder to stay under than the size. Good luck!

your problem isn't going to be size, it's weight. United doesn't have a weight limit. For Lufthansa it's 8kg which is about 17.5lbs. I've been on several Lufthansa flights where they look at boarding passengers and pick some who they invite to the desk so they can weigh their luggage; maybe they get a spiff for catching a bag that has to be checked ;-)

Are you concerned about only doing carry-on because of a tight connection time, not
wanting to pay baggage fees, or something else?

As Bob points out, there are a variety of potential conditions under which you might not
be able to bring your bag onto the plane. But the rule is that baggage fees are set by
the originating carrier, so if you meet United's carry-on requirements, Lufthansa cannot
charge you an additional fee for their flight.

However, they may require you to gate check the bag, which usually means you collect
the bag outside the plane after landing. But they should not charge you for doing this.

If all you care about is not paying the baggage fee, your existing bag should be fine. If
you want to be sure of having the bag with you at all times, then the newer bag may be
worth considering.

Rule of thumb....follow the rules of airline flying the plane.

It is my understanding that the major US carriers allow larger carryons without major weight restrictions than do most European airlines. If I were flying a European airline at any point, I would assume the lesser size and the weight restriction. However, if you're flying a smaller United plane from your regional airport, you may have to gate check your carryon.

they may require you to gate check the bag, which usually means you collect
the bag outside the plane after landing

No it doesn’t usually mean this. Gate checked bags usually are checked to the final destination, but not always.

Lufthansa told me, in writing, that unless something contrary is printed on my booking confirmation (indicating a special arrangement between carriers on that route) you must conform to BOTH carriers' requirements.

Everything else you read is speculation, hopes and wishes.

Lufthansa does indeed check size and weight and the requirement is smaller than United. They have clever little size/weight checking gizmos at the gates and I watched them getting used on 4 out of 4 Lufthansa flights this year. Random selection of people, but every flight.

I would just go with a 21" ultra light carry on with swivel wheels and be done with it.

In our dozens and dozens of international flights carrying on our luggage, we've only had to pay to check a bag once--Norwegian Air Shuttle. Lufthansa and United are not budget European airlines, and you'll do fine with your luggage.

Many of the airports of today have self check in kiosks, and no airline employee ever even sees or weighs carry on luggage.

I think those sizes will be fine and would not sweat it. I have had Lufthansa weigh by bag and gate check it, which was done for free. So I agree, weight is more important that a centimeter or two.

However, even with "self check in" terminals, often the machine only confirms your reservation and prints out a luggage tag and THEN you have to stand in a line to see a human and have the bag weighted and scrutinized. Happened to me coming and going on a recent Delta domestic flight. Kinda defeats the purpose but I suppose it speeds things up, but only a little.

It's even more complicated due to ticket class distinctions. If you haven't gone to the Lufthansa baggage rules website, here's the link:

https://www.lufthansa.com/de/en/handgepaeck#accordion-item-section-FreeOfCharge

Thank you all for your replies. I've realized that I may have been overthinking the bag size issue and defintiely overlooking others!

The reason for wanting to carry on is my regional flight to Dulles. I have a 1 1/2 hour connection and want to be more flexible if I miss the Dulles to Frankfurt flight. I asked about fees because I've spent so much already, but any fee would be a very small percentage of what I'm spending for the whole trip. So now I'm going to consider fees if I have to check a bag on Lufthansa as insurance that if things go wrong with the transatlantic flight both me and my bag get to Europe together. I don't know why but that seems better somehow. If I'm boarding the flight in Frankfurt I'm only an hour away from my adventure!

I'm going to take the bag that meets United guidelines and will be very careful with the weight. Thanks for the heads up on that! In case things do go awry and I'm separated from my bag and it goes on its own adventure. I have a plan. I'm taking a small backpack that I will use as a packing cube in my carry on for the essentials necessary to enjoy my trip. I can pull it out of my carry on, keep it with me and will have what I need to enjoy my trip, just with fewer wardrobe changes. Thanks again for your help!

I had to gate-check my US-sized carry-on flying ORD-FRA once. The gate agent said it was too big and then weighed it and said it was also too heavy. Since then for international travel I use a travel backpack that I've confirmed meets the size requirements for the European carriers.