1) just type in what values you want to on the custom map size - the largest I've tried is 1400 x 1400, but it stalled on me around 2350 or so. I've played an 800 x 800 to completion on both smac and smax.
2) change the alphax.txt value of a huge planet from its default to whatever you want. I have mine set to 360 x 300, so that when i hit "Huge Planet" as a choice, that's the size map that comes up (it also gets stored in your .ini file)
As per alpha.txt, defaults are all twice as wide as high:
#WORLDSIZE
5
Tiny planet|(early conflict), 24, 48
Small planet, 32, 64
Standard planet, 40, 80
Large planet, 44, 90
Huge planet|(late conflict), 64, 128
(that's where I've changed the huge planet values to 360, 300 - my "diamond" tiles are flatter in the grid 'cos of more tiles being forced into the vertical size)