Basic advice on creating a background for a webpage. The proper size and etc.

Let me start by saying this is my first project using either Dreamweaver or Photoshop.

I want to create a background for a website which two gradients on top of each other.
I guess my questions for the moment are, how can I ensure that the background I create will fill the page of most web browsers?

What size canvas is should I be using in Photoshop to ensure that I am able to do a good job?

I'm not sure I understan your question. If you wantthe backgroun. fill all the background you must use body {background-color: #0000;} for example or put the background-image and give it repeate. Two different gradient you put in two different divs - that's all.