The template format allows to specify the sizes with one of these codes: s (36x36), m (72x72), l (108x108) and xl (144x144). With +u (e.g. s+u) you can say that maps with underground are possible as well. Alternatively you can also specify template size in that format: 120x120x1 -> 120 width, 120 height, 1=underground. 0 is for overground.
Template sizes specify only min/max sizes which are allowed. To generate maps with “special” sizes you need to set size properties in CMapGenOptions. Via GUI you only have the possibility to select S, M, L or XL and that determines generated map size.
IMO we should improve map generation properties screen in the long run. But that’s another discussion.
BTW, is there any wiki page about RMG/template format? (perhaps we could collect a few infos there)