Which vehicle

Does this mean your trip is back on? My brother had an extremely negative experience with Sixt in Portugal. Refused to rent the prepaid car to him when he arrived at the pick up counter due to driver’s license and passport not being an exact match. They then refused to refund his money claiming he never picked up the car…….

I have used Sixt a number of times, iIn France (twice) and Germany and Switzerland and even in the US.
So my experience at least has been good, but no matter what car company I go with the real question is…what car?

And yes their is about a 66% chance the trip is back on. I will know by the first week of May or so.

The X5 is the easy answer. The A4 and 3 series have about the same dimensions—anyone over 6' would be cozy/tight in the back seats—especially if they're sitting behind 6'5" you.

The Q5 and X3 are the SUV equivalents of the A4 and 3 Series. I'm pretty sure they'd work. Don't know what else is on offer.

There are good YouTube car reviews that will cover space and capacity and give you a visible sense of them. (drivetrains may be different in European market cars). Alex on Autos and Tom Volk's Driven are probably the best for this.

I too am not a fan of Sixt car rental. They vary by location, but our one experience with them wasn't great.

At the Berlin train station, you've also got Enterprise, Avis and Budget. Your group might do well with a "mid size" SUV or Wagon over a premium priced Audi or BMW. You can compare rates on Expedia easily. Then go thru their direct website and/or AutoEurope to compare prices.

Speaking from driving experience (not ownership) the 3 series will definitely not fulfill your needs for space and comfort. On the other hand my 5 series sedan was a fantastic vehicle. I have no experience with the X5.

I am pretty tall (6-5) and one of the others is pretty big.

Pretty sure the A4 would be a marginal choice. I am 6' and getting in uses just about all my flexibility. At 6'5", you will have to be able to articulate in ways I do not. With you two in the front, anyone over 5'6" will be unhappy in the back
( A4 load limit is 1000 lbs)

I am able to get into the backseat, but is not where I would want to be for more than 30 minutes.

I would look at an SUV type vehicle , which will have better backdoor openings and more flexibility for bags

You will need a mid size SUV or (preferably) an estate car/wagon. We’re a family of 4 and we always rent cars. Last time we had a mid size SUV and struggled to get 4 carryons plus 4 backpacks in the back. A sedan will not work. You will end up with people holding bags on their lap. Look for something like the Opel Insignia wagon.

Audi A6 or BMW X5 for sure will meet your requirements but for a premium price.

Looking for a station wagon, VW Passat Estate or Skoda Superb Estate would be common in Germany, both with enough space for your luggage.

Last summer we were 4 adults for rentals in Ireland and Scotland with carry on suitcases and backpacks and we needed SUV’s no smaller than a Toyota Rav 4 to accommodate us and our bags. None of us are large people and the tallest was no more than 6’. When you say one of the others is pretty big, what do you mean? Are they tall or are they heavy?