Here is my opinions. I think everyone has their personal preferences. Not every party can satisfied your requirements. (And ugh, I don't like mixing the party of serious and just for fun members)
I only know two simple rules.
If I don't mind about items, exp, slow but lively, I'd go for big party. (Safer)
If I'm serious about everything, then I'd go for small party. (You might get scolded a lot
![Sweat Sweat](https://www.pandoraonline.net/forum/images/smilies/swt.gif)
)
Most of time, I go anything and I'll do my job properly. Who know? The party might be fun after all. :>