Is there a rule of thumb as to how large a citrus tree will get based on pot size, or what pot size is generally needed/desired to support enough nodes to get seedlings to maturity? I'd like to grow citrumelos, citrandarins, and citradias (sounds like an insect, doesn't it) on their own roots and hold them at about 5-6' and am wondering what the best pot size would be. I assume pruning will be important as well.

I'm not aware of any official container guidelines, ie formulas. You can support a full sized tree (or largest desirable in a container) with a 4x4x4 box container that requires forklift/crane to move. A 30" tapered container (the large boxed trees at a nursery) will easily support a 6' tree with very large canopy. Half-barrel containers can support large container trees. You can work down from there, even 18-20" pots can support the same as a half-barrel, but you need to water and fert more often and thus a bit more risky.

Don't prune seedlings, unless absolutely necessary... or at least until it flowers. Pruning seedling trees will delay flowering/fruiting. Searched Leaf Node Count on this forum. The exception is specific pruning to induce branching, like trunk pruning at young age.

I'm sure the experts will respond if you need more detailed info.

After re-read, it appears the container size question is really what is the minimum container (i.e. supportable root-mass) to support bringing a seedling to fruiting maturity?

Sorry, maybe Millet can answer?

As far as required pruning for the larger size, I still doubt it. The limited root mass will keep the tree limited in size; it shouldn't grow so far beyond what it's own roots can support that'd necessitate pruning. For long-term container care, you'd need to prune roots rather than foliage to keep healthy.