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Sorry for the confusion, I meant the duna multy maxi, at least I think that's the one, the larger one anyway! I do get confused with tank style cages as I don't use them!

75x40 is quite small really, I tend to think of about 80x50 (it may not seem much bigger but even that isn't that large once everything is in it) as a good minimum although all hams are different with what they like.
If you plan on putting the cage on a table, chest of drawers or something like that you can allow for some overhang as cages are heavy enough with all the substrate in to be plenty stable enough.
Have a think about what you'll need in the cage, a 20cm wheel (the right size for a Russian) with substrate underneath or a shelf of some sort to stand on takes up a fair bit of space, then you need a house, toys, sand bath.
If you are really stuck for space then maybe a DIY bin cage would be the best idea to fit what you have.