I've seen a few people struggle with their signatures looking blurry or getting cut off because of the dimensions. Usually, staying around 500x200px keeps things crisp without taking up the whole screen, and saving as a PNG definitely helps with the transparency if you're using a custom render.
If you’re trying to pull a specific element or a handwritten tag from an old graphic to use in a new layout, you could try using https://thetoolapp.com/image-tools/extract-signature-from-image/ to get a clean cutout. It’s pretty handy for prepping assets before you scale them down to the forum's limits.
Another thing to keep in mind is the file size limit - sometimes the resolution is fine but the KB size triggers the compression.