Accyweb avatar size is 100*100pixels.
Your sig is 400*100pixels.
The first sample I have done is 100*50pixels, which I think, looks the best I had to alter its proportions so it looked similar to the original.
The second sample is 100*100. Which as shrunk the width and kept the height.
The third sample is 100*25pixels I maintained the original proportions and reduced the width.
.