Here is all the info you need to calculate yourself:
The UPM (see File > Font Info > Font > Units per Em) will be scaled to your font size. That means if you set your font size to 30px and your UPM is 1000, then 1000u are equivalent to 30px; and 1px is equivalent to UPM/pixelsize, in this case 1000/30=33.333333u.
All you need now is to measure the distance between maximum height and maximum depth of whatever you want to fit into the given pixel size. The plugin ShowTopsAndBottoms can help you with that. Divide your UPM by this distance, and you have the factor by which you need to multiply your pixelsize. So: pixelheight*UPM/distance from top to bottom=font size in pixels.