[Imp]

I am pretty sure you can change in between battles there is a button called map or some such. 1 Changes default map type, 1 changes map size. If does not work prefs button is there to.
Cant remember what buttons are there but can also do following

If none of those work load a pregen map of size you want then generate a new one.

If do not decide to do till at the map screen the last option is your only one.