In noisy network environments with many clients, I generally enable RTS/CTS protection for packets larger than 500 bytes [1], and find it very beneficial. I believe because other stations and APs on other SSIDs sharing the same channel also wait.
The probability of collision is smaller with smaller frames of course, so perhaps the ‘optimal’ value would be a function of channel contention and number of stations/APs within 30dBm path loss. But 500 could be a good starting value.
Anyone has any experience/advice to share on this?
[1] http://wiki.mikrotik.com/wiki/Manual:Interface/Wireless#Frame_protection_support_.28RTS.2FCTS.29