If you wear fine quality shirts as I do (Thresher and Glenny, Hawes & Curtis, T.M Lewin, Stephens Brothers, Dunhill) you will have small pockets on the collars to take collar stiffeners. Some shirts will have flimsy plastic stays in them which will bend and distort after a few hot washes. Metal stays come in a range of sizes from 2 - 3 inches and I have tried them all. With very few exceptions 2 inches is the perfect length of stay for 90% of quality shirts. Longer stays will protrude from the pocket on the collar and distort it's shape. Larger size stays only suit shirts with very long (60's style) shirts. If you want them, I have stays in the larger sizes but I wouldn't recommend them - mail me for details.
eBay