To echo other posters, 100x50cm, continuous floorspace is the minimum for Hamsters. I volunteer in rescue (small furries) and we wouldn't allow adoption in anything smaller.
As exercise balls have been mentioned: they are cruel and should never be used. Hamsters navigate by smell and sight. They get disorientated and stressed in a ball, and covered in their own urine. They're banned in Germany for a reason. Free roaming, like a pen or sectioned off area of a room is kinder and more interactive.
Wheels should be 28cm and solid, so a Hamster can run with its back flat. A small wheel which causes it to arch, causes pain and back damage.
Thank you for coming here for advice. I know it's a minefield, with lots of Internet sites giving conflicting and sometimes terrible advice. As mentioned by previous posters, Hamster Welfare is an excellent site and the only one you need. All suitable housing options are featured there. Take their advice and you'll have a healthy happy hamster 🙂
https://www.hamsterwelfare.com/