With good casting form, you can use the lightest lines you can find. The size 3 lines I carry will work, as will TUSA's 2.5. The lighter the line, the more you need proper form to cast it, though. Heavier lines overcome a lot of casting flaws but the drifts are so much worse that the time spent on form is well worth t.

The smallest fly I have used without an indicator is #32, although I haven't fished one that small very often. I do fish a #26 a lot more often, but only on relatively shallow streams. A 1/16" tungsten bead or a single #10 shot got it down well enough. For deeper water the addition of a heavier fly or a larger shot would have been necessary.