I think a 50 gallon is one that some people use
- not sure on dimensions though. The 40 gallon breeder is 90cm by 45cm I think, so 50 gallon would be deeper as well. According to the link below, a 50 gallon is 94cm by 48.3 cm x 50.8 cm high. Or 37" x 19" x 20". Souunds like a good size, but isn't a massive upgrade over your current tank for the cost probably. The next size up that is deep enough looks like the 75 gallon tank which is basically a four foot tank - also 19" deep and 20" high. A four foot tank is a great size. If you got one in the next dollar per gallon sale you could get it for $75
Or ow about a detolf? It might work out a bit cheaper - and keep the 40 gallon for your new hammy
. The detolf works out at $60 though I think - you get a lot more length but not the depth and height.
Standard Tank Sizes
The one that looks like a great size to me is the 120 gallon one. It's just over four foot but much deeper and taller 124.5 cm long 63.5cm deep by 66cm high. Which is 49" long, 25" deep and 26" tall. The 60cm depth would make a big difference, and with the extra height you'd have space for deep substrate if you wanted. Not cheap to buy though as even with a dollar per gallon sale it would be $120. Great size tank though! I find the extra depth makes a big difference. Charlie's cage is 100cm by 54cm and the extra 4cm depth really makes a big difference over 50cm deep cages - floor toys just fit in better somehow and make the floor space more useable.