I'm a designer...I can do this!

I was on the free blog website and was reading blog secrets and saw "create your own background"! So I read on and it said you can create your own in a few easy steps. You need Photoshop; well of course I have that and Illustrator so I thought perfect! I'm going to have a custom one of a kind background. And if it's as easy as it says, maybe this will be a venture I take on; because I have SO MUCH free time! So I spent the morning designing the layout and I LOVE IT! So I saved it like it says to and the size it says to and thought "OK here goes nothing!" And it was way too small. I enlarged it and it's still too small. I have no idea why! If anyone has actually done this successfully, please let me know the resolution and scale it needs to be! Sadly I'm responsible for designing product worth 120 million a year but I can't do this!