1/25 scale - 3/4" or -12 AN larger fuel or oil lines.
1/16 scale - 3/8" or -6 AN - typically used for smaller fuel or oil plumbing.
1/12 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.
1/8 scale - Less than 3/16"
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 .035 Pro Tech Fittings
Each package contains one coil of .035 x 18", by Pro Tech.
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I believe Ted's braided fuel lines are from Pro Tech; in any case, I've found them to be be the best lines I've ever used. Following the instructions provided here, it's incredibly easy to produce 1:1 fuel line examples with the .025, .035 and .045 sizes.
When used with either Pro Tech or RB Motion fittings, you can get a professional looking assembly easily.