In string theory strings are around Planck length, but how big are the D branes in M theory? Can we see them with the LHC?
It's not really true that strings are Planck length. That's just the typical size of the low energy strings for which string perturbation theory works best. Strings can be arbitrarily large, in principle.
Similarly, branes can be of arbitrary size. They can wrap around compact dimensions, or fill all of space.