Hi Wendy,
Larger tritons do give a sound of lower pitch; so I guess your proposed small
shell horn would give a higher pitch, if it works at all.  However, I've never
tried it with a small shell, so I don't know if there is a minimum size that
will produce a sound.  I suspect there is.  A shell horn is blown like a
trumpet or bugle, not like a reed instrument, and I think there has to be a
large enough chamber for the vibrations to resonate, producing the trumpet
sound.
Paul M.