If you measure your lower spreader, and then measure in the full-size version of the linked photo below, the upper and lower spreader, you should be able to calculate the upper spreader approximate size.
US = (US photo)*(LS)/(LS photo)
If you don't want to go through that, just assemble the kite without the standoffs and lay the kite on the ground, then measure the upper spreader connectors distance and cut a spar to fit. You'll be close enough on that old school big boy.

