Height adjusting for background image

Hello all

I am returning to wordpress after a long time and i like the new features.
I have made a child teem from twenty ten, but i do not know how to modify the background image dimensions (not the header dimensions).

For the moment i use wordpress on localhost through bitnami untill i fix everything i need.

In the style.css of tweenty ten i have found this code

/* Main global 'theme' and typographic styles */
body {
	background: #f1f1f1;
}
body,
input,
textarea {
	color: #666;
	font-size: 12px;
	line-height: 18px;
}
hr {
	background-color: #e7e7e7;
	border: 0;
	clear: both;
	height: 1px;
	margin-bottom: 18px;
}

is this the section that need to be modified, and how can i modify.

Thank you for support.

Alin (zet)