I have a Laguna 18BX, has a 3hp/220 on it for 16" cut. I actually stalled it yesterday cutting 14" to the side of the pith. Wood had been laying outside for 2.5 yrs or so, was not real dry but not real wet. This was a first, have a fairly new 1" blade on it. Usually the 3hp powers through like butter but for some reason, did not like this wood. I usually cut 10-12 inch with no problem so if you can swing it, look for this horsepower. Thinking about it, I should have just used a chainsaw to cut this log down the center. For actually rounding a blank, it's a dream although the 1" blade will not make very tight turns, but a 1/2" blade cuts pretty tight.
Guess it depends on what you will use it for in the end. Splitting a log with a chainsaw and only rounding blanks, 6" or less my little Delta 14" with 3/8" blade works great, I think it's a 3/4 horse motor.