Some small guitars don't have a truss rod, but rather the neck is "sprung back" a little so it will to into the correct position when under tension. The Martin BP seems to be that way.

If that is the case, you are sort-of stuck with a small range of tensions.

So if use heavier strings, make sure you stay tuned down.

Also, the slack strings will wreak havoc if you want to play bottleneck guitar. The slide will push the string down to easily.

I think the Latin Americans (Mexicans?) have a tenor guitar. You migtht look into that.

Murray