These sigs might be small in visual real estate, but the file sized are getting rediculous.
The user guidelines warn against this, without giving an exact file size.
I believe on the old forums, it was <50K. You can do a lot with that amount.
Remember, that a lot of users, even on DSL, have annoying page loading speeds that are caused by pointless bandwidth.
For example, M3ta7h3ads sig is already at this artificial limit, while Smugs visually smaller sig is over 100K.