[A] and B are the concentrations of substances A and B in moles per volume of solution assuming the reaction is taking place throughout the volume of the solution. (For a reaction taking place at a boundary one would use instead moles of A or B per unit area).
http://en.wikipedia.org/wiki/Reaction_rate_constant
increasing the surface area DOES increase the value of [A] and B, it’s just represented differently because solids don’t really have a “concentration”
in other words; for non-solutions, generally represents an amount of reactant present for the reaction to occur.