Banners..what size are they usually? You know the ones guilds have made for under member's sigs?

Depends, there is not like 1 size everyone follows.
Just check out some different guilds and judge from that.

Usually from what i've seen about all guilds use one the length of the sing and fairly small..but Idk what that dimension is pixel wise.

Right Click > Properties > Dimensions: xxx x xxx pixels

it depends on the width of the sig. as ivan said, click properties to see this first

Well I didn't know about the properties thing before sorry =) Now that he explained it further i'll go do that.