This is all they say about cage size now (link on home cage from this web page)
"Hamsters need a lot of room (especially at night), so buy as large a cage as you can to provide your hamster(s) with suitable enrichment (such as small boxes and tubes – for example inner cores from toilet rolls – and wooden chew blocks) whilst still allowing generous space for your hamster(s)."
I think words like "a lot" and "generous" are too vague and open to interpretation. On the other hand the German welfare organisations are very upfront about a minimum size of 100cm by 50cm so it's not a bad guideline.
https://www.rspca.org.uk/adviceandwe...rs/environment