They're 6" now? How small were they when you bought them? Reason I ask, is that if they've only grown a tiny amount in a year and a half, they may not ever get too big.
At only 2' deep I wouldn't want anything over 12". The fish feed vertically and any bigger than that would be tough on the fish. And TBH one koi in that pond is one too many. I guess you could continually give your koi away when they get too big for your pond, and then get some new ones... But that kinda takes the fun out of it IMO.
The bigger question would be what do you want from your pond? If you're looking for a few pretty fish to watch swim around while you enjoy a morning coffee or an evening glass of wine you might want to get rid of the koi and put in a few nice comets or fantails. They won't get too big for your pond, and you can find some nice ones for next to nothing.
If on the other hand you're interested in Koi keeping, you'd best get a shovel and "get at 'er". There are lots of threads on pond construction and you'll get a good idea of a suitable size for Koi in short order.
Please don't get discouraged though, we all started somewhere, and in fact, my first pond was 1/3 the size of yours. After killing a few koi and doing a lot of reading I decided that I wanted a Koi pond. 3 ponds later, I have a decent environment for Koi and I couldn't be happier. The pleasure I get from it can't be measured. And the people on this board make it even better. Lots of good advice. Sometimes honest to the point of brutality, but spot on none the less.
Welcome to the forum!
Grant