Edit (originally misread): Good length for the surf is 6-8'. You want something short. If the line is long, The guy towing will pass the wave, but his buddy is going to get smashed on!
Well depending on the core of the line, length isn't really a variable. I think 8' is ideal though. Most everyone uses that cheap
polyester double braid which has a very low breaking strength, I've seen them snap. Off the top of my head, I think MFP line has about 18% stretch @ 20% breaking strength. So if I have a 8ft MFP line and I tow someone, it could stretch up to 8 1/2 - maybe 9 ft if your buddy eats a lot of Big Mac's. MFP line also floats like dinghy line. However MFP is so buoyant, that it can even float 2 stainless steel carabiners!
Its important that everyones tow ropes have a lot of stretch. My limiting ropes and tow loops have 0 stretch. They would not be a good idea for towing because when your towing a stand up aboard your stand up, you will feel every little thing and it can be a stressful ride back to the launch ramp. Not to mention, it can be hard on your ski as well. MFP + Dinghy line will be much easier on you and your ski.