The boostBarHeight property defines the boost bar's height on the background texture.

In other words this means, that the size is not in Widget space, but in texture space. If your background image has a width of 1000 and you size the Widget to a width of 250, a value of 500 means 500 * 250 / 1000 = 125 Pixel on the HUD.

This makes sure, that the boost bar always covers the same area on the background image no matter what size your Widget has (the background image will aways be scaled to the same size as the Widget.