If it's too big, vBulletin makes it smaller. Your avatar is 37176 bytes, Asamin.

Unless you're trying to use an animated avatar, there's no reason to need more than 120k. An uncompressed 150x150 32 bit image requires well under that figure. If you're using GIFs and you crop the image to the circular frame, you could easily have 7 frames in 120k. My 26 frame 500x125 animated signature weighs in at (well) under 120k.

As far as the size of the signature space goes, the 125px height was somewhat arbitrary, but it's been the limit for over 6 years now and hasn't been an issue. Before that, we had people with 600x480 sigs and worse.