I have the large size in Raffy's cage! I don't think it's that big really - it's tall and maybe a bit wider than the medium. The length is about the same for all of them. It depends what you want it for. I've always got the large size because it's an interesting texture for them to climb over - gives higher overhead cover, and tall enough to use as a ramp under a shelf. He has used it to run up onto the shelf but also just pulls himself onto the shelf at the front lol. Just to add it's not the only access to the shelf as it is quite steep (for an older hamster that is) - he also has the plastic tube run to get onto the shelf.
You can also push it down into the substrate to make it lower so I find the large size more versatile. You can't adjust the height of the lower ones. But it is wider perhaps if space is premium.
I have it part going under the shelf so part is a way of accessing the shelf (by running up the side) and it's partly a tunnel going under the shelf. The bigger ones also work well as substrate dividers if you want it higher on one side than the other.
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