I get the 10kg bag
It's not that massive and once you've filled the cage it would be easy to squish the bag under or behind something. I find that filling a 100cm cage from scratch takes up about a third of a 10kg bag of fitch. The small bags seem to do nothing really - doesn't seem equivalent at all somehow.
The 10kg bag is about the size of child's small beanbag chair - I used to just stick it in the corner of the room and put a throw over it (so it looked like a beanbag chair!). It comes in a large oversized plastic bag so there is room inside the bag to squish the contents into different shapes - I've had it stuffed under a chair or between a wardrboe and a wall before now!
It's definitely worth getting - saves a lot of money, is better than Carefresh in that there is no dust, and hamsters really like it. It's also hypoallergenic.
With a 100cm cage with 5" or so of Fitch in and a litter tray you could easily go 4 months or more without needing to change the substrate - just the odd bit of spot cleaning if absolutely necessary and emptying the litter tray. And even then because the cage is so large, most of it is clean and dry and you can re-use the dry half and mix it in with the new substrate.