To build off of what AR said, bigger is better when learning. I'd suggest 450/500 x 200/300. It's the size I used for some of my earlier stuff, and it got me to a respectable GFX point now. And with the SVGFX signature, I'd suggest opening up a small canvas, make text in it, in a unique and neat way, and save it. Then, you can save it as a .png and add it on to the finished gfx and they'll be consistent.