If you are social boating and tying up with others, at least 20' in the rear and 25' bow. (25' all around is simpler)
Nothing worse or more dangerous than too short of lines as boats pull in and out or trying to fip a 10' shoe string for the other boat to catch for tying on.
3/8 will work, I like 1/2 because it thows better and is more durable. Scuff a 3/8 or tie wrong and it can rub though quickly. Also a 3/8 will either cut or wind super tight into a prop. 1/2 will usually wind up, kill the motor and removal is much easier.
Double braided nylon. I am not a fan of Dock Lines or other other floating poly. Poly is prone to abrasion, weak and a floating unattended line is begging to be picked up in a prop.
While I am on my rant, 8" Big B fenders. No excuses, no compromises.
Thinking about it, I am in need of a Cove and a drink