Just a suggestion if you haven't thought of it yet: if you're looking to get some information before you design this sort of feature it might be a good idea to leverage RTB itself to collect information.
I certainly don't mean to tell you how to design a feature, I'm sure you've got more experience with this sort of thing, but I figured I'd share how I'd do it anyway. I would create a simple opt-in survey where people answer a couple questions about what kind of features and how much they might use this service, then automatically collect things like the number of saves they have and how large their saves folder is, so that I can establish a baseline for hosting size. Then you can use some kind of statistical brown townysis to determine how much free space each user gets. For example, if the average folder size is 20MB with a standard deviation of 10MB, then 50MB will comfortably hold roughly 97.7% of users saves.
You probably don't want to host the default saves since that's a lot of space wasted on saves everyone already has, so you would need to filter those out. There's probably quite a few people who don't have any saves, so you might want to use the median as a baseline rather than the average. Alternatively, you could filter out people who don't have any non-default saves since they won't be using the service and therefore shouldn't really get a say in the amount of storage.
