lol, don't worry. Please do comment back, so we know what you think and understand. This is a chattable thread, as long as it's on topic. Your last one, I would have pulled some of the green from his jacket out, and it looks like you squished him!! always try to not squish the render.
as far as sizes, do what you want, though in the real world, you'll have requirements to stay within. Almost all my sigs are 400x150, it's a size that suits my style.
Also try to sharpen him, he seems blurry and out of focus.
I see your thought process and I like it. Keep it up, and work on basics. You always want the focal to be the best part, and remember, the entire image DOES NOT have to fit in the sig!