phlogiston, Thanks for this -- my board is completely out of control with sigs. My mods will love you if this works! I am going to install it tonight.
Edit to add: Installed and works like a charm. As written, it adds a link instead of the image for sizes that are too large. Uncommenting the allowimgsizefailure error message logic gives an error message when trying to submit a new sig with a pic too large. Perfect!
Oh, and a note for those who tried this and said it didn't work, before you give up -- make sure that you are going through your UserCP to change your sig and not the ACP or MCP. Sig limit checking by the profile.php file is only done in the UserCP.