A rough equivalent of the Camry size-wise that's sold in the UK is the Ford Mondeo. (Ford no longer sells "regular" cars in the US aside from the Mustang – only SUVs and EVs.) You should all be able to fit, plus as Autoslash noted Avis is usually quite good about moving you into a different vehicle if the one you were assigned isn't satisfactory for some reason.

All that said, the rental-car industry still has a supply shortage – albeit one much improved from last year – and I'll use myself as a case in point here. My job also has a solid AWD code with a full-size Camry as the most economical on whole if you have more than one person. (I also get discounts on compacts and mid-size cars, but it's literally only $1 extra per day per category!) As it so happens, I'm in Vancouver right now; rented a car upon arrival from Avis; and ended up with an actual Camry – which was a surprise because it's only the second time out of 15 or so rentals over the past 18 months that I actually received a vehicle in my preferred category. Since sedans are an endangered species in the US & Canada nowadays, I'm almost always automatically bumped up to something like a mid-sized SUV.

The most ridiculous upgrade I've gotten is a GMC Yukon XL, an 8- or 9-passenger jumbo-size SUV that I'm pretty sure is the largest passenger vehicle sold in North America. It can fit three rows of people comfortably AND luggage in back, except I was not only traveling solo at the time; I was in Portland, Oregon, one of the most eco-conscious cities in America! Most people there drive Teslas or hybrids, but the Yukon was the literal only vehicle available. (I also ended up with a full-size Toyota pickup truck for a rental in Ohio last fall, and it's nearly as long as a Yukon XL – and definitely bigger than any Land Cruiser variant.

In any event, Camrys are great cars – still the best-selling sedan in the US, though it's been overtaken by the Toyota RAV4 as best-selling vehicle that's not a full-size truck – though if you have the option, I'd inquire about perhaps getting a Prius instead (which has more interior room than even the largest Toyota sedan, the Avalon, despite "looking" compact). Btw SFO is at least occasionally sold out of cars entirely (at least at Avis/Budget), but note that you can rent from any of their half-dozen or so city centre locations as well (which is what I usually do myself, since I'm thrifty and SF has a sizable tacked-on fee for rental cars picked up at an airport). There's one a few blocks from Union Square that's my usual go-to.