if i were to
use a
larger full range speaker would the fidelity/clarity of voice improve?

It depends on the quality of the new speaker. Larger size should give better
definition IF the newer, larger speaker is well-made and is in a quality
cabinet. But if the audio quality from the radio is already problematic (if you
can't get it filtered ideally for your ears) then it may be that no speaker
without other filtering or equalizing will give you better clarity than you
currently acheive.
But I'd guess that going from a 4" to an 8" or larger speaker would make some
real difference for you.
Linus