Bill,
Unfortunately there are no 'magic' lengths for your
loop. Something in the neighborhood of 520 feet should
work well on 160 meters, 250 - 260 feet for 80 meters.
The limiting factor is the 'MatchBox', it doesn't have
the impedance tuning range other tuners have because of
the small number of inductance taps it uses. (It's 'big
brother' has the same limitations, by the way.) The
'MatchBox' being a real 'balanced' tuner is nice, but just
not that big'a deal for all practical purposes.
In general (meaning that there are always exceptions) the
largest loop you can get into the air is the way to go, if
you cn 'match' it...
'Doc