bed size really depends on material. But if you dont wanna get a hook lift, for a 26k gvwr truck i would go 12-14' like said above those are the most versatile. I am not afraid to throw concrete in our flatbed dump, but its primary use is mulch and hualing plant material.
If i were to go 26k gvwr, i would get a 33k gvwr truck and get myself a CDL. 33k gvwr trucks are much more affordable because it requires a cdl, but isnt a huge truck. you will find an fl70 with 33k gvwr for a lot less than an fl70 with a 26k gvwr.