Okay... marble-sized then 🙂
(the actual size of the grain wasn't my point)
That's good because the size of the grain is not given by the developer type because that is a combination of the exposure and the developer time.
Developer types cause grain morphology, that is some developers change the physical shape of the grain giving an appearance of 'graininess' in the user when viewing at different print sizes.
Fine grain developer often promoted fillimentary growth that is the grains start out cubic and end up like this:
Grains don't clump together to form images either the grain centres have fixed positions:
Grain morphology creates tonal differences, and fine grain developers promote this.
Another myth is that sodium sulphite is responsible for fine grain in modern developers, it can help but isn't the main reason.
Sodium sulphite has a working action that is proportional to exposure, that is it attacks the less exposed grains allowing developer agents to access intersitial (interior) image and thus improves shadow detail. It has no effect on fully exposed grains.
Slow speed films are often single and only sometimes dual layer types so tonality is normally obtained using soft working developers.
Obviously then with grains in the range of sizes of a 25ISO film even Rodinal will not cause 'golf ball' size grain– no developer will be able to do that. Tonality will be a function of exposure and development time providing that the mix is either a soft working (probably metol) developer or a surface type like beutler or dilute Rodinal.