I just went and measured about ten of my pins, and the shortest is 2 3/4 and the longest is 4 1/4.

By far the most common length for the sword types is 3 1/4.

I would also recommend making sure that the pins are heavy enough to do the job intended. Light kilt pins are just a piece of jewellry.

I don't have any way to weigh the pins, but I like the heavy pewter ones the best.