What you could do, if you don't like diy (which I don't!) Is, for now, just get a really good sized plastic storage bin and use kitchen rack grids as a lid - a bit like this. A smaller cage isn't so bad if your hamster has lots of out of cage time every day - BUT there is a smaller cage and there is a cruelly tiny cage. At night hamsters are at their most active and need the space to run around - they are incredibly active at night. Also even a tiny dwarf hamster needs a wheel that is at least 6" in diameter or they can get back pain.
See the first and third pictures on the top row of this link - it is a just a plastic bin with metal grids (ie kitchen rack shelves) on top
The spacing on the grid isn't that crucial as, providing the bin is deep enough, your hamster won't be able to get to the top to escape. However, ideally the spacing should be 1cm or less - then you can have deeper substrate in the cage and the wheel would then be higher up - so they "might" be able to get out of the top then - so best to have 1cm or less spacing on the grids if possible.
https://www.google.co.uk/search?biw=...mg.cKXERzrUJSw
It needs to be enough space and also a solid base with a good depth of substrate/litter. The best thing you can give the hamster is litter to dig tunnels in
And a house that is dark inside to retreat to and feel safe - and a few cardboard tubes and tunnels as toys. Plus a wheel at least 6" in diameter. Making a playpen as well sounds great