Actually, ProE does know it, but I don't know a reasonable way to get to it. As I suspect you know, the info Model Size is the length of the diagonal of the part's bounding box. The bounding box can be found in the file header. Look for:
@bounding_box 33 2
2 33 [2][3]
$BFADA2572936909,BFADA2572936905,C0221DA25729369
$400076895CA4DA41,401C3B44AE526D21,3FADA2572936905
You can see this is the coordinates in hexidecimal of the two corners of the bounding box.
But, don't know how you could use that to get what you want.