i had mine up for about a week, been running lcs for quite a few days, and both actively use other spaces (like this one) and post a lot in the ones on my instance. currently i'm sitting at 4.5 GB for pictrs (not a typo, that's what the picture server is called), 2.3 GB for the postgres database, and 5.4 GB for docker. total disk usage is about 14 GB for now, i expect it to grow in the future but idk yet how fast it will be. people are reporting about 100 MB a day since the reddit migration, and tbh that might check out.
if you're hosting it on your own nas you're probably gonna be fine space-wise. i'd just recommend to layer a vpn and/or a cache in-between -- i don't know exactly how to do this, i went straight for the cloud route, but i have seen people in !selfhosted@lemmy.world doing that, and the lemmy admin matrix chat is nice too.
just fyi, your instance does have to be reachable on a domain if you want federation to work. also, keep everything you can on the defaults and only change things one by one, the error messages are not very helpful. i spent like a day trying to debug why lemmy wasn't starting up at first, turns out i just had an instance name longer than 20 characters.