I've generated a batch of 10 random maps which feature all the settings which have been discussed on forum to date. I've picked the "best" one, which is kinda nice and strategic to me.
Most of the maps in the batch were kinda meaningless, lacking a strategic configuration. These are the low-level maps of the batch. Then, there are those maps which are a midway between the "meaningless" and the "meaningful", which have been chosen for the preview picture below. These are not that bad, but also not very comfortable or strategic to play on, but yet they give a vague idea of how the mastermap could look like. Just, imagine the features of these mediocre maps combined together, and mentally correct those ugliness immediately visible at first sight, such as that too large blob of land lacking topological features (corner region of the bottom-left map), or that internal sea which is a little too large to hold its full strategic potential (upper-left map). The outcome you're imagining might closely look like the mastermap.
The size is 21500, computed as 250 * players / landmass * 100. The formula
proposed by monamipierrot would have resulted in a larger map (26000), which I thought to be unnecessarily larger.