Rodinal most certainly is not a fine grain developer, it has the properties of giving very sharp edge to areas of different tones 'and this includes the actual grain of the emulsion. Hence the obvious grain you have seen It is called the 'Sabbatier' effect. Rodinal was an 'acutance' developer You get very much the same situation with digital if you verge on over-sharpening an image. The pixels (AKA film grain) become more obvious on large prints and there is a tell-tale fine line around areas of different tone.
You should be able to achieve a 9,5x12 print without the grain becoming too obvious using FP4, and with Pan F, 12 x 16 is more than within it's capabilities. Just don't over develop.
You can demonstrate sharpening effect very clearly by taking a picture of pylons on a sunny day using a red filter, The cables from the pylon and the insulators holding them will have a very fine clear line around them. The effect is more pronounced if you use Rodinal at a high dilution instead of 1-25. One of the instructions that came with the original Agfa Rodinal was minimum agitation. 30 seconds continuous, then one inversion every 30 seconds until the time was up. This caused the developer at the edges of different tones, especially the dark ones where it was more active, to become exhausted more quickly and cause the sharpening effect. You can minimise this effect by using conventional agitation - 10 secs continuous at the start and every minute thereafter, but the effect will still be there only less obvious but with a slight reduction with grain and you will also have to modify the development times.
There was an Ilford developer, almost certainly a version of Rodinal called Hyfin, only available in powder form where the development was 18 mins for both Pan F and FP3 (in those days before FP4) and a stern warning not to use it with anything faster! It has not been available since the 1960's and oddly enough Ilford will not release the formula to the public or bring it back, which is a pity because it was a lovely developer. The sharpening was as good as Rodinal, but gave less grain