You've got things confused.#mainWrapper {width:950px 61px 0 58px;
Imagine a box. Padding is what keeps things away from the inside of the box, just like packing something in a real box where you add padding to prevent the object from moving around inside the box.
Width is the size of your box from side to side. (You don't specify height, usually, in CSS. The height adjusts to fit whatever is in the box automatically.)
So specifiy your width in a line separate from your padding, like so...
Margin (to make the lesson complete) is the space outside the box between it and any other objects. Margin is often used in CSS to push things over or down where you want them.#mainWrapper {
width: 950px;
padding: 0 61px 0 58px;
}
And to be clear: everything in CSS is inside a box, whether that box has a visible border or it's just implied. So you can do these things -- set width, apply margin or padding -- to pretty much anything on the page.
HTH
Rob



