does anyone know what the size is for creating your own wallpapers...

Tech Specs say it's 352x416 but I always just use the Nokia Image Converter that comes with PC Suite.

And does anyone find it strange that there aren't any tech specs to be found in the manual?

it's not that... you have to take into consideration, the 2 options on the bottom of the screen and the top section...

352 (w) x 300 (h) works OK...