When I looked at the XML for the maps and I believe the tactical maps it seems to me it defined them in terms of size and number of sectors:
<DisplayName>T_Beach_Arctic_01</DisplayName>
<MapTileWidth>16</MapTileWidth>
<MapTileHeight>16</MapTileHeight>
<MapSectorWidth>1</MapSectorWidth>
<MapSectorHeight>1</MapSectorHeight>
<SectorTileWidth>16</SectorTileWidth>
<SectorTileHeight>16</SectorTileHeight>
<SpecialMap>0</SpecialMap>
I would imagine by this then that one only need change these values in order to change the size of the tactical maps - if you want to look at them more yourself look at them in data/maps/tacticalmaps for more reference info. If I recall correctly there is also a similar set of instructions in one of the core XML files that may be used to define sector size so I would at first try sticking to the standard sizes. I also believe based on what I have looked at in the XML that all you will need to do to add new different tile types is define them like it does in the XML :
<TerrainTypeInfo>16,Tactical_Forest</TerrainTypeInfo>
Of course since I wasn't around in Beta perhaps this has already been covered and won't work.
