I seem to have discovered a method of creating hi-res sprites.
First, create your sprite as normal, so for a frigate as an example, it should be about 110 pixels tall.
Use the ship editor, create the ship, get it working in game.
Boot up photoshop, or the image editor of your choice.
Look at the dimensions of your lo-res sprite - find the size in centimeters
create the hi-res version of your sprite
go to image size, set the dpi to 400, then enter the size in centimeters of your lo-res sprite. The image should end up slightly larger than before.
Save the image as a .png, overwriting the lo-res .png.
Bam! Hi-res sprite.
I've tested this in game, the sprite is exactly to scale to all other ships, both in campaign, battles, and variant editor.
I've just no idea why.
So...did I discover this? Or is this an incredibly obvious thing that everyone knew about?