Antenna length
Your receiver antenna length is not going to matter very much as long as you are in the ballpark. 2 different receivers I have on 22, and 38 I measured are both 39 inches. Of major importance is in antenna lengths on your transmitter. That one must be to spec for the frequency your transmitter is tuned to otherwise some of the rf power will be fed-back to the output transistor. Depending on the mismatch length the transistor will run hotter than normal and could eventually destroy itself. Not having the tx antenna out full will give you less rf power being broadcasted therefore less range. Nevertheless I always measure my receiver antenna and scratch the number on it's case. In case of a mishap and 1/2 the antenna go missing I can always add the proper section back to its original. Wire thickness on the receiver antenna does not matter as long as it can take the physical abuse of flight and hangar rash.