Standard Road King shocks are 13 inches. What you want to look for, is the Street Glide shocks. They are 12 inches and with a 1 inch lowering block will put you back to stock 11inches. Any of the shocks will fit it just depends on what ride height you want.
I got lowering air shocks from Jireh cycles sold under the Ultima brand name. They're 11" shocks and I love 'em. I think the set was $120 when I got mine a few years back, i'm sure they cost a little more now, but still a great price for them being new. Otherwise the popularity of the roadking name on the shocks is mostly because they're the easiest to find on ebay for a used set and can be had very cheaply.