For a given length of wire your inductance will be greatest with an inductor
that has a large diameter and a short height. However if you make your coil
to short you can get problems with flashover from end to end.
Wire is reasonably cheap compared to capacitors and NST. Using a lot of wire
is the easiest and cheapest way to increase spark length. I would recommend
using 22 gauge on a 12 inch by 4 ft concrete form.This will give a nice low
frequency and if (when) you upgrade to using more NST (or a pole pig) you
will be all set. Finer wire also works and I would not be surprised if one
could drop down to as fine as 28 gauge for a pole pig powered coil. But fine
wire does not seem to survive strikes very well so we use the thicker stuff
as insurance. (tiny wires are hard to wind, they get crossovers easily)
Jared Dwarshuis