Unlike the coins of today, Roman coins had substantial weight, especially the brass and copper coins. The largest coin, the sestertius, was significantly heavier than any modern coins. Although the coins were not standardized since they were struck by hand, the brass sestertius had an average weight of about 25 grams (0.88 ounce); its diameter was approximately 30 millimeters (1.18 inches) and it was quite thick, at 4 millimeters (0.16 inch). The weight of the next largest coin, the brass dupondius, was a little less than half that of the sestertius, at 12 grams average (0.42 ounce), with an average diameter of 25 millimeters (0.98 inch) and a thickness of 3 millimeters (0.12 inch). The dimensions of the copper as were nearly identical--average weight 11 grams (0.39 ounce), diameter 24-25 millimeters, thickness 3 millimeters. When new, the as could easily be distinguished from the dupondius by its red color, but the coins acquired a patina over time and were thus difficult to tell apart, hence the later adoption of the convention of showing the emperor with a radiate crown on the dupondii and wearing a laurel-leaf crown on the asses.
The precious-metal coins were considerably smaller. The gold aureus had an average diameter of only 19 millimeters (0.75 inch), but its thickness (3 millimeters or 0.12 inch) and weight (7 grams or 0.25 ounce) were substantial. The smallest coin, the denarius, had an average weight of only 3 grams (0.10 ounce), with an average diameter of 18 millimeters (0.70 inch) and thickness of only 2 millimeters (0.08 inch). The Roman unit of measurement corresponding roughly to our gram was the scripulum, 1/24 of an uncia, which is similar to our ounce. A Roman scripulum is calculated at a little over a modern gram (1.137).
Be aware of these dimensions in VRoma, since your purse will only hold a limited number of coins (especially sestertii)!
The above coins were photographed with kind permission of the Museum of Cultural History, University of Oslo.