How do I make a map of a custom size?

[Reveilled]

I've decided to put myself through the painful process of recreating the Colonization 1 Americas map again, but it's been years since I last made a map, and the first step completely escapes me.

I need a blank canvas that's 58x72 exactly, and have no clue how to make it. Anyone know?

EDIT: Nevermind, found a workaround of sorts by editing the Carribean mapscript to generate a map close to the right size for me.