70' is standard but I like to adjust the length according to the wake. Some like it rough, some don't (especially learners and doubly especially learning kids).
The water is rougher up closer to the boat. Calmer farther away. We would barefoot on a 90 ft. rope. Put two together. Wakeboarders want a bigger curl, so we come closer to the boat to get the best curl. If you learning on combo skiis, give then lots of rope, and even if they go outside the wake, the wake is small. Let us know what you find.
Well then I guess I buy a 50 foot rope and a 75 foot rope and see who gets up and who drinks the lake water. That will be the bench mark for the rope length.
Side bar. I have skied behind a 10 foot home made built boat with a 15 horse tiller engine. When I went wide, I could move the back of the boat. Back then I weighed 155 lbs.
Took my 14 year old son skiing for his first time today, he got up on his 3rd attempt. After a few times he was able to handle turns and pretty much keep going as long as he wanted. There was somewhat of a consensus on this board that 75 ft was a "standard" length tow rope so that's what I bought. No complaints, it worked for us.