The size and dimensions of your signature are fine
If you are ever in doubt of what the sites rules are with regards to signature sizes:
Quote (Site Rules)
Signature Rules and Guidelines (violations may result in loss of rights to have a signature):
The entire size of your signature shall not exceed 400 pixels wide by 150 pixels high.
Text sections of your signature shall not exceed 6 lines of text if you include no images, and 3 lines of text if you have an image.
Total size of the images contained in your signature shall not exceed 70 kilobytes.
Yours is 300 x 86 and 16.63 KB - which is well under the limit.
In addition, your avatar is fine as well. The dimensions of avatars will always be reduced to the sites standard (so you don't have to worry about that), however they must also be less than 50kb (yours is 6.88kb).
If you have any further questions, please let me know
