http://www.n6mrx.com/Antenna/Cubical-Quad1.htm

Thanks for the link. The calculator works for me.

One remaining Q: how do I take into account the "pigtails" of the driven
element? A nice square loop is nice, but I've got to connect it to the
(relatively) big diode, and this requires a bit more wire from the loop to
the diode (on the + side of the loop) and to the meter connector (on the -
side). Do I include these wire lengths in the loop size calculation? Or
ignore them?

Or does the 1000pF cap effectively terminate the loop (re. size)?

Thanks.