We've used your server for IRC blitzes quite a lot Gandalf. And thanks for it.
Random maps are fine. Sometimes we prefer maps with balanced starts so that doesn't work as well random. The random maps can have some really awful starts, but that's something you work with.
CBM is the big thing. It seems an even bigger thing with blitzes so that the big bless nations don't totally dominate.
In my view, the big thing with easy research is to help the game end in a viable amount of time. Any harder, and a blitz takes too long for a single sitting and it's pretty much impossible to get the same group together at another time. Anything that helps keep blitz time down to 4 hours is good.
It would be nice if you did a 30 province random for duels. With water that's probably viable. Maybe even 25. The 50 is too big for duels given the time constraints.
I imagine you don't want or can't use fixed maps so that's no big deal. The randoms are fine. Sometimes you get a good blitz and sometimes you don't but the point of blitzing is to try out stuff fast. So scrapping a game after turn 20 because someone got a great start and is clearly winning is fine with me. If there's time, just start another.
The majority of the blitzes end in stalemate or concession. I've yet to play one to the clear end. Then you get all the IRC arguing about "I was winning" "No you weren't I was" etc

Which is part of the social fun.
I'm not sure which games you find fill up fastest, but in terms of IRC blitzing, CBM with easy research would be the most popular. And the 25 and 50 size. Some days we get 2 blitzes going but lately with the holidays it's been hard to organize.