Actually the most common size is probably 350x150 and it looks like you just put text on a picture. Not really a sig. If you used a stock (GFX lingo for picture) then post the original so we can see what you did. I'm 99% sure you used a stock on this, but if you used a render (another GFX term for cut out image) good job for your first. But I doubt you used a render. You kinda have to understand what good sigs are made of before you start making them. Just look around in here for a week and see what people are talking about. If you don't understand what they mean when they are rating other art, then ask questions no one will be "mean" to you.

No either get gimp online for free or get a torrent of Photoshop those other programs can't compete. Also the original looks better it looks like you just added some random blur and made the color snot green lol. Then you put some huge unmatching text on it. Take my earlier advice seriously.

You should be making the backgrounds lol. Unless you're talking about getting c4d's and brushes.