"Do you think you couldn't cut a 1 root with anything less than a 14-16 blade?"
Kinda depends on whether my assumption (based on info in the first post, and a fair amount of experience) that the root is only PARTIALLY clear of dirt/rocks, etc, and down in a hole -
I have and use 2 recip saws, a corded super sawzall and a battery DeWalt, and a few blades that are close to long enough (at least 10", haven't checked 'em lately) - I know how to do a plunge cut, but it's NEVER been fun to do that with that long a blade, they're too flexible NOT to go sideways on you when trying to get started. Even a shorter blade tends to rudely push you backward in a blind hole. Going slow (and a blade that's DESIGNED for plunge cuts) can help some.
Also, unless you could get all the way around that 1' root it'd be an exercise in frustration to try and cut enough away from only the top with a short blade...
All in all, the recip wouldn't be my first choice from the given description; I also have several chain saws from a little 10" electric pole saw up to a 32" Husqvarna, several circular saws from 6" to 10" and several other types that would be ridiculous to even consider for this job;
All of which is why I'm with the chainsaw guys on this one - if the OP is experienced enough to safely do a plunge cut (and stay away from that rock) even the chain might survive with being sharpened afterward.
IF not, a replacement chain is BUCKET-LOADS cheaper than HIRING anybody with any sort of tool... Steve