When I specify an extended source the dimensions that appear in the 'sketch' are much larger than I would expect. E.G. if I put in a 'flat' extended source with 'major axes' of 8", the sketch shows a disk 32" in diameter. It may be that the calculation attempts to convert the major axis value to a semimajor axis value, but multiplies instead of divides by 2. In this example the disk should be either 8" diameter (if 'major axis' is just that), or 16" diameter (if 'major axis' really means semimajor axis).