I'd probably go with a 1.25 inch to mimimize the mass of the bit. In other words, just a little taller than the thickness of the workpiece. You can get much longer bits and they will work, but that's a lot of steel to be spinning at very high speeds, and the stresses will be higher if you take more than a little off the workpiece with each pass. I also don't like having any more exposed cutter than is necessary. I like an excess of caution. My take anyhow.