[Kureo]

Really depends on what type of thread design you are going for and what kind of information you want to fit on it. If you want to fit a lot of pictures along side with pictures, you may want a bigger one, but if you just want some text you may want a smaller one. Depending on your style as well, however I can give you what I normally use, but I would recommend you to experiment it a bit with it first.

900x1400 - Regular Shop Size
800x1200 - Quick Information
800x600 - Paragraph Information

I wouldn't recommend you going higher than 900 width or lower than 600 Height as they will get really small or too big beyond those points.