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Hey Mike, looks like a quarter wavelength antenna should be about 9 inches in length but I haven't had a chance to attach to the transmitter/receiver yet and check the results.
In the wiring diagram for step 7 "Adding a Radio Frequency Transmitter", it looks like the blue and red wires going into the transmitter should be swapped around. The "ATAD" (or 'data' spelled backwards) pin is the left-most pin on the transmitter in that orientation, so the blue wire should be connected to that. I also found I needed the transmitter/receiver within inches of each other to finally get the "A" character received (may just be due to circuit board orientation on the breadboards I'm working with), so I'll have to experiment with antennas. I can't wait to use some of this in my Halloween setup!