Practically speaking, I'd say signature size should be kept within these limits:
1. About 250-300 pixels in height.
2. An aspect ratio which is mostly like... well, a signature banner rather than a portrait. The former looks somewhat tidy, the latter just looks too big almost regardless of the image size.
And... I've never heard of signatures not appearing in IE. Perhaps it's cause you're not logged in there? I hear XenForo hides signatures for guests...