I wonder too how far he can stretch. Ive seen him stretch pretty far, but never anywhere was it mentined how far he can stretch. Maybe if Marvel releases an official handbook of them (even though it'll be about the 616 versions) we can find out if they mention it there.
Mister Fantastic possesses the ability to convert the mass of his entire body into a highly malleable state at will. In such a state, he can stretch, deform, expand, or compress his entire body or parts thereof into any contiguous shape he can imagine. He can extend his limbs, torso, or neck to great distances: the maximum length he can distend before his body segments become painful is about 1,500 feet. (Although he can extend discrete body parts, such as a single finger, an ear, or an eye, he seldom if ever isolates such parts in his elongations.)
The Thing possesses superhuman strength, enabling him to lift (press) 85 tons.
Now that I think about it, Reed and Pym have completely different biologies. Reed doesn't even have any internal organs, I'm not even sure if he has a skeleton!