Rick Priestly has posted recommended 28mm formation sizes for his forthcoming Ancient rules titled Hail Caesar.
Single figures are assumed to be based on 20mm x 20mm square bases. Cavalry on 25mm x 50mm bases. Elephants 60mm x 80mm. Light Artillery 40mm x 60mm. Heavy Artillery 60mm x 100mm. 2 Horse Chariots 40mm x 80mm & 4 Horse Chariots 80mm x 80mm.
Formations come in sizes of Standard, Large, Small, and Tiny. Aka Black Powder.
A Standard formation will consist of 16 to 20 models in a two rank formation. So you can have a front rank of 8 models, 9 models or 10 models. It's your choice but it has no effect on the game.
A Large formation will consist of 32 to 40 models in a two rank formation. So a front of 16 to 20 models.
A Small formation will consist of 8 to 12 models in a two rank formation. A frontage of 4 to 6 models.
A Tiny formation will consist of 6 models in a two rank formation. A frontage of 3 models.
Phalanx and Warband are recommended to be in four rank formations with frontages as above with double to amount of figures.
This is only a small summary of the two page document. For a full version you will need to join the Hail Caesar Rules yahoo group.
I got excited and order some more Gripping Beast Romano-British to make an impressive and sizable army.