That depends on the marina, some charge for "set" lengths, (eg if you are on a 50' pontoon in a 30' boat you pay the 50' price, some are willing to charge by the foot, so check out the places you fancy & ask the question.
That depends on where you would like to go ! if you want to "do" all of the system then a 57' ish boat is the recommended max, 60' if you want to do the L&L (though some have done it with a bit of faff with longer) It's a "how long is a piece of string question ! Depends again on your aims.
Suggest you look at some other boats, & decide what you can live with !
Sorry to be so vague, but as there are so many variables it's is solely your choice, get dozens of boaters together & you will find just as many differences of opinion & boat layout !
Alf