@madeleine_ Hoooo boi...I honestly don't remember, but usually I do a full arm span (fingertip to fingertip) and a few extra inches/centimeters, but since this is a thicc and long one, I recommend adding at least an extra foot/meter or two just to be safe.
Also, certain strings will become shorter than others faster in most Celtic patterns, so just keep that in mind. If you're still not sure, don't hesitate to message me. Hope this helped! 🙂