According to Paul Erdkamp, "War and State Formation," in A Companion to the Roman Army, p. 102:

Each man received a ration of about 830 grams (1.8 lb) of wheat per day in the form of unmilled grain.

In Caesar's commentaries there aren't any specific indications of what each legionary got per day, but he does mention having to find food quite often. In addition, inopia frumentum (lack of grain) is often given as a cause for revolts and wars. Also, it seems clear that the legionaries received MORE than just grain, because Q. Caecilius Metellus Numidicus ruled in 109 B.C (about 50 years before Caesar's campaigns) that troops HAD to roast or boil their meat. If they didn't have meat, such a rule would obviously not have been issued.

However, if you're looking at time periods after Caesar, the Codex Theodosius (7.4.11) says that soldiers in 360 AD received hardtack biscuits (buccellatum), bread, ordinary wine (vinum) and sour wine (acetum), salted pork and mutton.

Cassius (130 - 175 AD) ordered his troops to carry laridum ac buccellatum at que acetum, which means 'pork fat, hardtack, and sour wine.'

In Suetonius 'Divus Iulius' section 38, he does mention that Caesar gave certain food bonuses to civilians. This does not, however, reflect upon the diet of an average legionary:

To the people of Rome, besides ten modii of corn, and as many pounds of oil, he gave three hundred sesterces a man, which he had formerly promised them, and a hundred more to each for the delay in fulfilling his engagement. To all this he added a public entertainment, and a distribution of meat.