I am buying new halyards (online) and I want to ensure that I am buying the right diameter... is there a "best practice" here?

Yeah I can eyeball the lines, but when we're talking 7/16 or 1/2 inch and I'm looking at old line... well the old Mark 1 eyeballs might be off a touch.

Is there a better way than just replacing the lines with what used to be there? I'm also going to be replacing clutches... so I don't mind resizing the lines.

The main is 190 Sq Feet, the Jib is 238 Sq Ft, and the Staysail is 84 Sq Ft.