Email .sig size limits

Here's what might seem like a minor issue, but we have a customer who is rather upset. It seems he wants to create a signature using webmail, but can't believe he's getting an error message limiting him to 64kb. (I have no idea why he needs such a large .sig - all of mine are 1-2k, max.)

After way more time researching this than is considered cost effective, we can't seem to find what the size limits are for any of the webmail portals, nor whether they can be changed.

Any info on signature size limits?