Bar Lines

You can make barlines invisible by selecting them and pressing "v".
But if you select the correct time signature Musescore will place the barlines exactly where they need to be.

What is your situation that makes you run into this problem?

see https://musescore.org/en/handbook/measure-operations#split-join

Split and join is definitely the way to go if you are trying to write something in free meter. What I'd recommend is first using join to create a measure bigger than you'll need, entering notes until you reach the point where you want it to end, then using split. No worries if you don't make it big enough the first time, though, just join up some more as needed.