Even though hamsters are very small they actually require a lot of space, it is not uncommon for hamsters in the wild to run 9km in one night. The Veterinary Association for Animal Welfare is Headquartered in Frankfurt and was established in 1985 by 150 veterinarians, also known as (TVT) it “recommends the owner give the Syrian hamster as much space as possible — at minimum 100cm x 50cm x 50cm (L x W x H) which is 5000 cm2 (775 in2)” as can be seen on Wikipedia. The minimum recommended cage size is for ALL species of hamsters including dwarf, on the Wikipedia dwarf hamster page it also says “A lone, or pair of Roborovski hamsters should be kept in a cage ideally 50 x 100 cm.” Please note this is the minimum of unbroken floorspace (not levels), the more space the better.
Don’t be fooled by what the pet shops recommend. Pets shops usually always sell cages which are far too small, this is because they are often cheaper for customers to buy and they save on shop floor space. If you buy a cage which is too small, your hamster will sadly live a miserable life.