My take on this:

1) If you're going to play backup for multiple singers, or for fiddles, mandos, or Gawd forbid, b*nj*s, and you do not have some kind of amplification, either mic or pickup, on your instrument, with the ability to adjust the mix, you're probably stuck with a dreadnought, large jumbo, or small jumbo/grand auditorium/0000/x14/whatever, as a minimum. Although I'd like the chance to try a Collings 12-fret slothead 000 in that situation. It might just be able to manage.

2) The 000 or OM is much more comfortable to play, especially if you're short, if you are...well, not skinny, and if you play standing. I am told that this is even more the case if you are a woman of the opposite sex, who has been generously blessed in the area of secondary sexual characteristics, since a dreadnought lacks the narrow waist where the left one can...well, you get the idea. Also, that size probably has the best balance for solo playing or accompanying solo singing - especially the 12-fret slothead models.

If I had to get by strictly on only one guitar, but I could have it built from scratch, I think a 12-fret, wide-neck, slothead version of a grand auditorium size and shape (Taylor x14 or Martin 0000 or J) might be the way to go. But it's more fun to have a few to chose among.

Peter.