i am trying to figure out how to create a char array with an unknown text size
You can't do that.
I want to add a name after welcome and of course the name is an unknown length
You'll need to determine a maximum length that you want to support. I can't imagine that you want to welcome
SomeGoofballWithARidiculouslyLongNameThatMeansAbsolutelyNothing to your program.
You would have problems fitting that on the LCD anyway.