Lots of recumbent riders use shorter cranks, to encourage spinning on hills, as we can't stand and use our body weight to climb. I use 155mm and I'm 5'8". Some sources: PBW bikes has a 155mm crankset in 130/74 BCD that comes with 30-42-52 rings (about 95 bucks). Origin 8 makes a 130/74BCD crankset without rings, available on Amazon for about 35 bucks, and Mark Stonich of Bikesmith Design shortens cranks as well as having already shortened sets for sale. My wife has a crankset on her recumbent in 28-38-48 160mm crankarms. I bought the crankset for $7.99 new, removed from a youth bike. It's not elegant, but it does the job.