I've got the Surly Disc Trucker and, as others noted, the 26" vs 700c tire size comes with the bike based on the frame size. I'm short (5' 4") so I have a smaller frame with 26" tires. I've ended up not doing any loaded self-supporting tours yet (ended up doing a couple of week-long tours but my wife drove support so didn't need to carry everything).
I have used it as a back-up road bike and did a couple of century rides on it. Over the last few years, I've also gotten into gravel riding so I pulled the Surly into service for that. No doubt, it's a tank and may not be the best bike for those kinds of rides, but it has served me well enough. While heavy, it does have the lower gearing which I think helps offset the added weight.
Having said all that, I don't think I would get the Surly Trucker if my goal was long-distance road riding (vs loaded touring). Probably would look for something more targeted to long-distance riding where you didn't need to carry a big load. But, if you want to get just one bike, and you know you want to do self-supported tours along with gravel rides and long-distance rides, IMO, the Surly wouldn't be a bad choice.
On the 26" vs 700c decision, as a recreational cyclist, I don't think I can tell much of a difference. The thing I have noticed, especially with regards to using the bike for gravel rides, is there does seem to be a smaller range of tire choices in the 26" size,