You can follow Q009's suggestion of making the resolution higher by simply using bigger chunks. So when you make the width and the height 4 times as big, simply use 4*4 pixel chunks instead of individual pixels. That way you can keep this oldschool look you seem to go for.

I have the resolution this low because that's the actual size they should be on-screen. Also, the default crosshairs are all 32x32. I tried resizing them anyway to 128x128 but it didn't work. It would be nice if crosshairsize -1 would render the exact crosshair without any modifications.

Perhaps you should ask eihrul for that (we do not have direct influence on the development process of the game neither the engine).
For now, just crank up the resolution and detail.