I don't think CMs have size limits.
Anyway, I don't really do the CM contracts or anything. If you wanted to join my CM in hopes of getting some "accidental" CM trading done to boost your score (when you buy from a CM member through the world market, it counts toward your CM trading even though it was really just normal trading), you're more than welcome. There's like 8 countries and 2 enterprises with maybe just under 1000 corps. I think one enterprise did 3% of its trade through "accidental" local market trading last month.
Mostly, I just like having the name there. Think it over. If you want to join one for the sake of being a member of one, give "Law of Identity" some consideration. Probably best to treat this as an option of last resort unless you'd really like to see "Law of Identity" as your CM name.
If you join another for the contracts, be sure that you're careful with CM contracts... generally, you only want to be contracting stuff if you're using an ASQ strategy or building "contract-only" stuff or self-supplying high Q weapons.
As far as other economic stuff, I can build or relocate corps to your countries regardless of whether you join my CM or not. I have two enterprises after all.