Just a newbie question, at what point in size or speed do you generally make the jump from 60 to 70 foot lines? Or is it more complicated than that? Having flown nothing bigger than a 40 and about 55 inches of wingspan, I was curious.
10 feet is FAR too big a change to make just to adjust the speed. A foot is a pretty significant change.
For reference, most current competitive 40-75 sized models are somewhere in the range of 63-65 (67 or so being the limit as mentioned above, since I am talking eyelet-to-eyelet measurement). As far as where to start goes, for a typical 650 square inch piped airplane I would suggest 64 and then adjust from there - in maybe 1 foot increments. I used to carry them in 6" increments, but now I just have my usual lines (64') and a backup set, and then a 63 and 65 each.
For a 20/25FP or LA airplane, in the range of 400-500 square inches, I would suggest starting around 60-62. For a Fox 35/McCoy35, maybe 56-58', and carry some 54 and 55s to test with.
Brett