I think he already refered to the OE-Art Selection thing "(Further than Size and Hieght)".
You can edit them further but as far as I know, those are only editable for the host of the edit. In otherwords, wont be effecting multiplayer options. Open the WC3 MPQ and you'll find 4 types of Selection rings, Regular, Large, Dotted and Large Dotted (the last too is when you are sharing units from another player and thats the rings the other player would see). They are Splats, and thusly you can edit their image to look different.
It wont work for Multiplayer since its a change to your WC3s core files and your files only.