scmJS docs

DIM

The map's size in tiles. The sizes of MTXM, TILE, MASK and ISOM follow from it. StarEdit offers sides of 64, 96, 128, 192 and 256 tiles.

Name DIM␠ (44 49 4D 20)
Size 4 bytes.
Read by The game.
Required Yes. The game does not load a map without it.
More than one copy Only the last copy counts.
In scmJS Read, and written again when you change what it holds.

Layout:

Offset Type Holds
0 u16 Width in tiles
2 u16 Height in tiles