"Size" utility conflicts with @page rule.

Take a look: http://www.w3.org/TR/css3-page/#page-size-prop

This:

``` stylus
@page
    size 20em 30em
```

produces this

``` stylus
@page {
  width: 20em;
  height: 30em;
}
```

but should not.

I suggest to rename "size" utility with for example "sizes" or something else.
