Size doesn't really matter for collabing. You need to try and build friendships and acquaintances and through that collabs will naturally form. If you're looking to link up with someone just for the sake of a collab, try to find people within your range of subs, so it is beneficial for both of you.
0 subs is enough to start trying to collab. The thing is you normally collab with similar sized channels and/or friends. If you have 100 subs expect to collab with other channels with around 100 subs, dont expect someone with 100K subs to collab with you.
Just a word of warning... Getting people who say they will collab versus people that actually do can be vastly different. I'm over 4K subs, and still haven't been able to get anyone to fit the bill. It can be very frustrating, but that shouldn't dissuade you from trying. Just be prepared that it may take quite a bit of time to get what you want.