1/25 scale - 1 1/4" or -20 AN - radiator lines, main fuel cell line or other large transfer lines.
1/16 scale - 3/4" or -12 AN - typically used for coolant, large fuel delivery or dry-sump oil.
1/12 scale - 1/2" to 5/8" or -8, -10 AN - typically used for fuel, coolant and oil plumbing.
1/8 scale - 1/4" or -4 AN - typically used for brake line applications, small oil lines, some small fuel line applications like nitrous, pressure gauges and vacuum lines.
The scale size is determined by its outside diameter (OD) compared to real world braided line since an inside diameter isn't useful in scale modeling. See the tables in the section called LINKS, INFO & TECH.
Use Pro Tech Fittings sized at .060"