I know how to change the poi icons, that part is much simpler than changing the size. I've gone thru every bit of the file with resource hacker, in the icon group section it has the file size listed and its possible to overwrite but it doesnt save when you save the .exe . AFAIK there is no other way to change the size and there's no point to change the icons if they're gonna be the same size as the existing ones.
You can change the POI images and I think you can change the size.
I would start by looking through resource hacker.
The down side to Iguidance's POI is there is no way to insert your own information.