It is probably best to keep your page within the norms but I am no expert about that. I think most people deal well with 900 to 1000 pixels.
One problem you might experience with iweb it that not all templates can be changed easily. The image that makes up the page background might not work well if you go from 700 to 900 pixels. In that case you can get into it and edit the image in your favorite image editing software and you will be fine. You may also have to edit the top and bottom image.