I've kept Robo's in a cage as big as a meshed Ferplast 140 - and they loved it lol. I got fed up with the cage as the mesh meant I couldn't see in lol but the Robo's were very happy in there. I had a shelf going the full width of the cage at either end and plenty of tunnels / houses etc and they used every inch on the space lol.
However, when one of the pair died of old age the other had to be downsized as she got a bit senile in her old age and couldn't cope with the space and was moved into a Mickey XL (80cm narrow barred cage) then as she got more 'doddery', bless her, she ended up in Mini Duna. It was easy to see how much happier she was in that smaller cage for her last 3 months or so. I think you can soon gauge as to what they're happy with.
Plus it justified me keeping a variety of cages so I was able to prove to my husband that they were needed!
The levels / tubes on the Duna Fun were really annoying though. My dwarf hamster used to wee in the tubes which meant a lot of cleaning! If you got it I'd recommend getting 'end stops' so you can block off the holes if necessary. It can, of course, be converted into a 'normal' Duna.
These days I'd much rather go for a Maxi Duna or a narrow barred cage of some sort.
ETA: that's so true Cypher, I had a Chinese hamster in a bigger cage and he hated it no matter what I did! He ended up in a cage that I hated because it was hard to clean and I had been going to sell it as I didn't feel it was long enough either but he loved it so he stayed there until he died. It was a 3 tier IMac Fantasy.