fishinnstudyin:

It's the diameter that's most important. The butt section of many leaders is often between .019 - .024. For a 6wt I'd go with between .019 - .022 and I'd probably make it 12" long or less.

Monofilament fishing line in the 17lb - 20-lb might be OK too, I'm just not positive about the diameters.

Note: I just checked some 17lb Suffix I bought for one of my baitcasters and it is .016 so maybe you want 20lb test.