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Max, as pigslo mentions all the rope makers have sizing charts. They'll also gladly work with you on the phone or by email.
The size of a rope isn't everything, a better quality fiber or different manufacture can make a thin rope stronger than a thick one. You need to figure out the loads you expect on your lines, and then pick lines that can safely carry that load. "Working" load and breaking load being two very different things, also.
Dock lines should have some stretch in them, so they can absorb the snaps and jerks of bouncing around at the dock. And in time, they will wear and need replacing, so buying a larger spool and putting half away may actually be cheaper in the long run.
Don't be surprised if the lines you select dont' fit properly under some cleats and hardware--it is not uncommon for hardware to be sized smaller and cheaper than it should be. If you can't fit the proper lines under the hardware--replace the flimsy hardware, too.
Some folks secure a boat, really secure a boat, every time they dock, figuring you never an be sure what weather will come through before you get back to the boat. Others are more casual about everyday versus storm prep. That's a choice you can make too, there's no need to use docklines that will hold you in a major hurricane all the time. (You just need to be sure you can and will get back to the boat as needed.)
And of course, if you're out cruising and tying up in other places, it never hurts to have some extra lines stowed. Some old ones that you don't mind using on tarred pilings, some longer ones if you need to float in tidal ranges, etc.