Dave,
In a nut-shell, a larger antenna will tend to be more 'receptive'
at lower frequencies. Then you can think about matching the
impedances of the antenna system and receiver.
Larger antennas have the 'problem' of being 'larger', as in where
do you put the thing? Reducing the size of an antenna can be more
practical mechanically, but tends to be less practical electrically
(noise, less 'receptive', etc, etc.). Your 'best' bet would be to
find a reasonable compromise between the two kinds of 'practical'
thingys (mechanical/electrical). Usually easier if you can make it
bigger, sort of.
- 'Doc

(all puns intended, even the ones I didn't intend)