I hated the tiny little spell decals for my spell gems and wanted to make the bar the same as my buff icons (30x30). So this is what I found out about the spell gems.
In the EQUI_CastSpellWnd is a simple change to make the decal for the button 30x30 but you can see in the EQUI_Animations Its not so easy when you need the pictures to all line up. Originally the settings were 24x24 for size and 256 for picture size. I changed to 30x30 and 320 for same ratio and everything worked fine without changing the gemicons01/02 size. As long as the ratio is always the same like 48x48 and 512 you can change the decals that make up your spell icons.