This forum is for threads involving discussion only. If you simply want to ask a quick question, or make threads such as links to videos or quizzes then post in the Off-Topic discussion forum! Me myself I like 300x100, Cause I got used to it on a forum where that was largest allowed.
i think it varies on the picture/render your working with and how you want it to show in the signature... i used to use 300x90, now i use 400x120... anything is really workable tho, just some different widths/heights can affect render placement and the overall feeling of the signature due to stock/render placement.
Most of the people have their own sizes, usually between 300-400x100-200. I use 350x125 most of the times, i think it looks good that way.