The Kadee couplings for NEM pockets are #17, #18, #19 and #20. #21 requires a draft box and not an NEM pocket. I've found that #18, #19 and #20 are best for my layout which has 3' radius curves. Not found a use for #17s yet.
Best thing to do would be to buy one packet of each and try out on your stock to see what fits best.