Here are a few samples of very quickly done signature banners, with dimension and sizes included.
420x64 (5,722 bytes)
320x52 (4,015 bytes)
300x64 (4,064 bytes)
300x40 (1,757 bytes)
300x34 (3,097 bytes)
240x64 (4,964 bytes)
240x40 (2,860 bytes)
All of the above JPG images were saved at 50% compression ratio.