Don't you guys think that if you use like 400x100 it's just too thin/narrow? i can't understand why so many people do that. my renders don't fit too well if i make a sig that size. The most narrow I'd probably ever go is like 375 x 100 (if the "y" is 100)
I find 350*130 to be a perfect size, anything wider leaves me with a bit too much space, anything less wide leaves just enough room for the render, and if i make it any shorter theres not much room for a render.
Normally Depends on what i feel. When i started out GFX it was about 350 x 150 now i go down to anywhere between 340-380 x 80-130
I usually go and start with a file with a size of 500 x 200. And then afterwards, I'll get my crop tool and I'll crop it to a standard size that I want. So in other words, I have no exact size for it. lol.
It all depends on the theme and style in my opinion. For smudging, something with a small height but large width, for manipulations, something that has a ratio of width 5:2 height. (500*200 px would be great) Hope this helps.