So if I just used explicit pixel sizes, such as the following, I'd be fine?
-webkit-background-size: 48px 48px;
background-size: 48px 48px;
By the way, since we're on the topic of media queries and background-size, the caniuse chart says that IE6-8 doesn't support background-size. I know those browsers also don't support media queries. So, is it safe to assume that any browser that supports media queries is guaranteed to support background-size as long as pixel are used for the dimensions?