from colmar to chamonix rental car

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You will need a van so it’s unlikely you’ll find one in Colmar

I see a van at Avis in Colmar, but although it says it seats 7, that only leaves room for 2 suitcases. You will need something bigger, like a Renault Trafic. Can you drive a manual? One available at Basel airport on the French side.

This past summer, we rented from the Europort (Colmar/Mulhouse/Basel/Freiberg) Airport, on the French side, from Europcar. There were just two of us, and they upgraded us to a small convertible for no extra charge. I can’t tell you what they’d have in larger vehicles, but I’d definitely check with Europcar. You might know this, but if you’re wanting to return the car/van in a different location than where it was picked up, there might be an additional drop-off charge, so determine that, too, whatever rental company you go with.

We rented a car in Colmar to drop in Chamonix in 2021. There were 4 adults and they gave us a really small car. I didn’t reserve a compact, but that's what we got. It looked like a clown car trying to get our 4 carry-on sized suitcases, 4 backpacks and ourselves in the car.
We stopped at an airport rental location somewhere along the way to see if we could get a larger car, but it was futile. (We stopped in Lyon before going to Chamonix.) We just had to pile in and make it work. We have some hilarious pictures that make me laugh out loud every time I look at them.
Driving a large van presents its own challenges. A couple of times we were really glad the car was small. If I were you, I'd rent two cars.

hey hey msoy12
when are planning this trip and is this group all adults? you have received info on different sites to research.
you will need a van which can be cumbersome/awkward. it's not all about seats but room for all your luggage for 7 people, room is bare minimum in trunk and space behind seat.
don't know where you are from: USA/Canada/Europe but please do research if you need an IDP (international driving permit aaa.com/IDP). you may or may not, all drivers need one, that and your home drivers license together when driving. also read laws, rules, signage when driving. plus picking up one city and returning to another may charge you a drop-off fee. you may need a sticker to drive thru switzerland roadways?
i'm in the same mindset as celeste, rent 2 cars.
aloha

msoy12,
Be aware that what we call a compact, or a family-size sedan, or even a van, is not the same as what you find in Europe. They are not necessarily trying to con or scam you, but car sizes are just smaller. The titles aren't the same as in the US. There were seven of us, 5 adults and 2 kids, 9 and 12, in June of 2023 and we rented two cars.

And Princess Pupule has good suggestions.
Bon Voyage!

You might have more luck renting between bigger hubs. Like Strasbourg to Annecy or Geneva (French side of the airport). You'll need a Swiss vignette to drive through Switzerland (the shortest way).