Which program to use is up to you, I personally rely on bad ole MS Paint (
) for icons.
First thing to do to keep the game standard is to open SHPed, save a bitmap copy of the palette (load WinSPMBT_Palette.pal, click "display palette" then "save collage") so you know which colors are supposed to be used.
Then open some other ingame icons for inspiration in visual standards (slopes, lighting, colors, shading...) and the background color.
Oh, and the scale is 6 inches (152.6mm) per pixel for vehicles and 3 inches per pixel for infantry and artillery.