I enjoy playing lots of small games. First, some of the randomness makes the games very different (e.g. in
a gigantic game, you've got a good chance of getting resources, but in a small, you could have only one military resource on the whole map - and you're enemy's parked a base right on top. An even better reason is my comp's under minimum specs
As for custom challenges, it just depends on how masochistic you are:
Protege Conquest - choose one minor race, give them all the help you can, and get them to conquer the universe without attacking anyone yourself.
Bring It On - you're evil, and you set all the aliens
to pure good. You have to go to war with them as soon
as you see them. With any luck, they'll ally against you and make things interesting.
Just pulled those out of my arse, but you get the idea.
Personally, I think the point of custom games is to force yourself to try something new. I don't care about the difficulty level, whatever works for you - though doing stunts like that on maso obviously earns you lots of bragging rights - new medals would be nice too...