If things are made of particles and they can be divided, you must come to an end at some point. So the particle which cannot be divided any further is an atom. If that atom does not have dimensions, then it will be so tiny, it will have no extension, no breadth, height, with anything. No matter how many atoms you put together, you'll still get an infinitesimally small point. It's the same problem which we had with geometry. A point is a this geometrical entity which has no length, no breadth. But if you have that and then so many number of points makes a straight line, ti, that's logically not possible.