Working in photoshop, then plugging into CSS is a common way to do a site. I used to do that, but now I kinda do them both at the same time. I find my design always changes from photoshop to the real thing just because it looks different, so now I just make the images in Photoshop and as I make them I put it into the actual web page with the CSS.
I think the norm (if the width is static) is 1000px wide, so it fits well on a 1024px wide screen. Height is basically whatever, it just depends on the content. If you want your width to vary depending on the screen size, then you would use percentages when defining width in your CSS rather than a defined pixel width. Then again, this is just the norm, people don't follow the norm
all the time.
If you want it static, but to also look good on many screen sizes, you could also use @media (in your CSS). You should be able to use that to change what the layout is depending on the screen you get. I think that is mainly for people who want their site to work on mobile phones along with a regular computer screen.