The "bible" AC 43.13 says...
...that you should use the largest size that will fit consistent with the hole in the bolt and the slot in the nut (section 7-127)
But, in the case you mention, a -208 might or might not fit.
A 5/64 (the -208) cotter pin is a nominal size of 0.078 inches.
The hole in the AN4 bolt is 0.076 to 0.086.
Given the range of tolerances involved, the -132 cotter pin (0.063) would be a consistent fit, whereas the 5/64 pin might, or might not fit. It also might be also affected by the "off center" tolerance of 0.010, so even if the larger cotter pin did fit, it might not line up with the nut slots.
In your example, I would say the -132 would be correct....
UPDATE - the newer MS24665F specification which replaced the older AN380 cotter pin specification says to not use the 5/64 size pins for new designs.
http://www.everyspec.com/MS+Specs/MS2/download.php?spec=MS24665F.007213.PDF
The 1/16 pins are recommended by this MS specification for a 0.078 size hole, so definitely, the -132 pin is the correct one to use....
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