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I totally agree that it can very much depend on the individual hamster & some are happier with more space than others but I generally keep my Russian & Chinese hamsters in cages that are 80x50cm, I have kept both species in 100x50cm cages when they clearly enjoyed & wanted more space.
I find it very difficult to provide enough enrichment along with a good size house & wheel in anything under 80x50cm but how you set up a cage can be just as important as size, most hams don't like too much open space & all enjoy a really deep layer of substrate to dig & burrow in.
Which cages are available will depend on what country you're in but I find the hamster heaven (without the penthouse & tubes) to be a good basic cage. My favourite cage is the Marchioro Kevin 82 but sadly it's no longer available in the uK.
I would say that you are unlikely to find a good size cage in a pet shop & the better sized ones are usually found online.