I feel that its easiest to not discuss maps in terms of their overall province numbers. Talk about province-per-player. Do you feel that a game is best when everyone can take the provinces around their castle, and then have to prepare for war? Or would you rather some wilderness between players. The more wilderness there is, the more it affects the play of some nations.
Here is part of a page called "Map Thoughts" on
www.dom2minions.com
For a player to capture all of the provinces around his castle as a buffer and for resources, we are probably looking at.. 5? There are 17 nations (at this time) so 5*17 would be 85 provinces. Actually, if you fairly consider the land and water nations then the best map should probably be 15*5 or 75 land provinces, and 2*5 or 10 water provinces, to a total of 85.
Down near the end it has some recommended terms ...
SMALL 5 each, or 85 total provinces. Best done as 75 land and 10 water.
MEDIUM 10 each, or 170 total provinces. Best done as 150 land and 20 water.
LARGE 15 each, or 255 total provinces. Best done as 225 land and 30 water.
HUGE 20 each, or 340 total provinces. Best done as 300 land and 40 water.
EPIC 25 or more each, maybe to the maximum 500 total provinces. Something like 450 land and 50 water.
And it has links to various places you can get randomly generated maps. Kindof ugly ones but playable. They are done to fill in the gaps if people want to play a new map of a certain size. Especially the really large ones.
Of course these sizes are all figured to allow for all 17 nations to play at once. For fewer players, consider the math rather than the sizes. For only 10 nations a small map will play like a medium sized game. There is alot more discussion on the "Map Thoughts" page
EDITED: Hmmmm not the best batch of maps today. But tomorrow there will be a whole new batch. Or if you want me to I can kick off the generator to randomize them again.