Given the numbers of cavalry and infantry, whether 4000 or 5000, in each legion, and given likewise the depth, length, and number of the troops and companies, the dimensions of the passages and open spaces and all other details, anyone who gives his mind to it can calculate the area and total circumference of the camp.
If there ever happen to be an extra number of allies, either of those originally forming part of the army or of others who have joined on a special occasion, accommodation is provided for the latter in the neighbourhood of the praetorium, the market and quaestorium being reduced to the minimum size which meets pressing requirements, while for the former, if the excess is considerable, they add two streets, one at each side of the encampment of the Roman legions.
Whenever the two consuls with all their four legions are united in one camp, we have only to imagine two camps like the above placed in juxtaposition back to back, the junction being formed at the encampments of the extraordinarii infantry of each camp whom we described as being stationed facing the rearward agger of the camp.
The shape of the camp is now oblong, its area double what it was and its circumference half as much again.
Whenever both consuls encamp together they adopt this arrangement; but when the two encamp apart the only difference is that the market, quaestorium, and praetorium are placed between the two camps.
Walbank Commentary