OK, the upper limit is set to 2mb.
but try to keep it resonably low(1mo or less). considering that I was able to keep the banner, which is twice the sig size and 66frames long, in about 1,1mb, you should do it with your sigs too. if you reach around 2mb, you'd better have a reason to.
you shall all taste the pain of gif optimizing! ;p
also, for some help, you can check out this tutorial