IMHO the physical size of sigs id a bigger annoyance than file size.
If a file is huge, then it wont load for ages, therefore i can keep reading what im interested in.
If there is a bloody huge image every 3 posts, then it gets tedious to keep scrolling through.
Banner style sig picks should be the accepted format, like recoils, and manike's.
as to text sigs, short and sweet...
youve gotta thank your sponsors, and make a quote or two.
but thanking your mum for giving birth to you, and the nice man down the post office for sending your paintball stuff to you... please.