Divide the length value by the width value and there you go..
Maybe combining Min and Max with the ratio thing would be something.
I think it'd give something like this:
code:
<SkinInfo>
(...)
<Dimensions>
<[Some window]>
<Size>
<Width>
<Minimum>X</Minimum>
<Maximum>X</Maximum>
</Width>
<Height>
<Minimum>X</Minimum>
<Maximum>X</Maximum>
</Height>
<WidthHeightRatio>1.333</WidthHeightRatio>
</Size>
</Dimensions>
(...)
</SkinInfo>