Hi there. It's annoying when pet shops sell you a cage that is too small. But you might be able to get a refund if you haven't had it long. Some people have done this. Ordered a new cage (as Cypher says, Zooplus has some good sized hamster cages at reasonable prices), and then taken the old one back saying it is not big enough/suitable for their hamster and got a refund.
It sounds like the one you have has quite a small floor area, but quite tall. Hamsters do better with as much floor area as possible and not too much height as they can fall and injure themselves. They like climbing but aren't very good at getting down again and don't see well, so tend to just let go sometimes.
RSPCA minimum recommended continuous floorspace is approx 80cm wide/long by 50cm deep and height between 38cm and 45cm really. Higher than 38cm and you need to think about shelves and setting things up so they can't hurt themselves if they fall.
They also need a minimum of 3 to 4" depth of substrate, which some smaller cages don't allow for if they have a shallow base, so a cage with a deep base is best. Ideally 6" or more. This cushions any falls and also allows them to do hamstery things like digging tunnels and rearranging their substrate.
If you couldn't get a refund, then the smaller cage could be good as a play cage or somewhere to pop your hamster when you are doing a cage clean
These are the Barney and Alaska cages mentioned. They might not look as much fun as some of the colourful cages with tubes, but can be made fun quite easily with toys and hamsters love a cardboard tube to play in and chew.
Alaska Hamster Cage | Free P&P on orders £29+ at zooplus!
Barney Pet Cage | Great deals at zooplus!
Our hamster is in the Barney cage and it's great. You can fit a lot of stuff in it and it comes with a good shelf and a good sized house for a hamster. He has loads of space to roam and hide and is quite happy in there and never seems desperate to come out! Although he does enjoy out of cage time as well.