Also don't be afraid to use solid brass, steel, and aluminum wire from companies like K&S Precision Metals. It comes in many diameters and the thinner diameters works very well for wires and small diameter hoses. It bends easily, holds its shape, and is easily painted. .015" brass wire works very well for representing 3/8" air lines, even if they need to be coiled.
For tubes and hoses I use, .010" material for 1/4", .012" material for 5/16", .015 material for 3/8", .020" material for 1/2", .025" material for 5/8", .030" material for 3/4", .035" material for 7/8" and .040" material for 1".
Fuel Injection tubes I use .012" brass to represent 5/16" lines.
Small air lines I use .015" brass to represent 3/8" lines
Large air lines I use .020" brass to represent 1/2" lines
For fuel supply lines I use .025 or .030" to represent 5/8" or 3/4" hoses.
For Hydraulics I use .080" material to represent 2" suction hose, and .040" material for 1" pressure and return hose.
For electrical wiring I use all sorts of material, braided fishing line, monofilament fishing line, sewing thread, brass wire(this includes stranded conductor wires from small wires)
Like I said in a previous post I use the cord wires from a Play Station 2 controller for trailer supplies. It's outside diameter is .035" But on a wire that small the individual stranded conductor wires are .006", a few of these strung together make for a convincing wire harness. .006" in 1/25 scale is around 3/16".
I'm all for eyeballing but plug wires the size of heater hoses bug me.