I only had success (in terms of a suitable icon size) where I clicked on an individual icon to get to the "large icon" screen
(like this -
Download Free Envelope Package Glossy Waxed Wood Icon ~ Icons Etc. - Notice how much bigger it is when you click it again and it pops out)
then, I clicked the icon again to open a pop-out window with the icon. Right clicked, saved as, and copied to SD card. This resulted in icons of an acceptable size on the Evo desktop.
Here's another good icon site, although, again, you have to be careful with size (I think the minimum useable size is 128 px by 128 px - the sizes vary a lot, so make sure you check; also note that sometimes an icon will show that size, but the icon itself will be smaller than the surrounding transparent box; you need the actual icon to be 128^2 or larger).
Iconfinder | Search and download free icons
Here's an example of a set where the icons are a sufficient size to work well on the Evo desktop.
Iconset
utdated icons | Search and download free icons | 1