A speaker's enclosure determines what it will 'sound' like to a great extent. Assuming the same speaker, a larger enclosure tends to mean more bass sound, a smaller enclosure a more 'tinny' sound. Lots of variations in that, the size of the enclosure certainly isn't the 'key'!
Since most voices are 'mid-range' then a 'mid-range' type speaker tends to reproduce them pretty good. It doesn't have to have a lot of 'range', the average speaking voice doesn't have a lot of 'range' either.
I have a computer speaker (just part of a speaker system) that's in a box about 6 x 6 x 8 inches. Maybe a 4" speaker(?). It does pretty well. I've also bought a speaker and just used the cardboard box it came in as it's enclosure (put some material over the front to make it 'pretty' of course!), it did pretty good too, till I finally ripped the speaker's cone.
And then there's the thingy about my ears aren't going to the same as yours. Best suggestion I can think of is to just try them until you find something you like the sound of. That shouldn't cost more than a couple of hundred, right? Yeah, right.
From what you said, it sounds like you have a 'tone' control on your radio. Remember to try that too, when you try out all them other speakers.
Good luck.
- 'Doc
(no help at all, huh?)