You want about 20-30 feet, depending on how long your pocket is. Keep the boat end of it plenty long and make a few ties. You can always move it in or out as you get more comfortable.

I don't think a knotted rope is any more dangerous than a small or large handle. You can get tangled in any of them if you are not careful or they are just too long.

If you are using a regular (small) rope, try tying some tennis balls or solid foam balls near the end for a better grip. Otherwise an old-school tow rope is going to be murder on your hands as you learn.