I was going to say that the 20 gallon long might be a bit narrow (12 1/2 inches deep) and also go with the bin cage idea. That way it doesn't have to be too big to start out with until you guage the personality, but still has good floor area (ie good depth) the shape can affect what you can fit in and layout as well.
So Amity's suggestion of a 40 gallon (breeder) sectioned off, might be best and more economical in the long run. It's only 6" longer than the 20 gallon long, but is a good depth (45cm or 1 foot 6"). And again you could have one end (maybe 1//3) that is a deep digging area to make the floor area smaller overall. Plenty of cover and things to hide under would help as well.
Or a cage. I know Cypher uses cages for hers. But I know cages are expensive in the US.
There is this perspex tank as well - it's not huge but might be good as a starter tank until you know what kind of personality they have.
The top one on this link. It's about the same length as the 20 gallon long but the same depth as a 40 gallon breeder.
HAMSTER CAGES
I think if it was me I'd either go for that or a 40 gallon breeder sectioned off with a third of it as deep substrate and lots of hides and tunnels. But I haven't actually owned a Chinese
