The largest main belt asteroid is 1 Ceres, which alone contains almost a third of the total mass of the whole main asteroid belt.
Ceres is large enough to be in hydrostatic equilibrium, i.e. its own gravity is strong enough to pull it into a roughly spherical shape. Since the mass of a spherical planet scales as the cube of the diameter (assuming constant density), piling all the other main belt asteroids together onto Ceres would only increase its diameter by a bit under 50%. It would still be a roughly similar type of body — a small sphere of partially differentiated rock and ice, with no atmosphere to speak of (as it'd be way too small to hold onto one).
Thus, to answer your question, a hypothetical planet containing all the matter currently making up the main asteroid belt would look pretty much like Ceres already does, just a bit bigger.