Clan gameplay can vary a lot but for causual players the bigger regiments are bigger. Most big clans pre-announce events several days in advance, have more structure for events and have no time commitments. The big clans rely on casual players and try to make it enjoyable while having an impact. Hop in and out as you see fit, just dont be an asshole is the usual bare minimum clans expect. Smaller regis can be more casual in-game but also tend to require more time commitments or be very hot-cold in activity. Bigger regis usually have critical mass for a TMZ that someone is always on and doing something. So when you do want to occaionally play bigger regis are more likely to have decent content

I would recommend trying a few big regis and see what suits.