If you're open to changing developers, I will suggest Xtol 1+1 or 1+2. Much less grainy than Rodinal, but with less loss of resolution/sharpness from solvent effects than you would get with D76. You'll also get at least one stop more film speed. It's definitely a different look than Rodinal and you may not like it, but it WILL help reduce the obtrusiveness of the grain. Or you might like it; who knows.
Using Xtol doesn't make the grains smaller, it reduces their intensity. If you think of grain noise as being a wave (variations in density with respect to position), then Xtol reduces the amplitude of the wave, not the wavelength.