Greek obols, Judaean yehuds and their fractions are so tiny, sometimes only about 5 or 6mm in diameter and 1/8th of 1 gram!! How did people carry them around, and reasonably use them as currency? Why would they do so?

Coinage as currency was not invented until the 7th Century B.C.E. For centuries and millenia trade generally conducted using a barter system. The barter system was based on the value of goods vis-a-vis the materials they were exchanged for. During this early period precious metals were also exchanged as a commodity, based on weight. Coins in essence were a sophisticated way of assigning a predetermined value to precious metals, so that their weights and value would be standardized,negating the necessity of weighing.

It is for this reason that the earliest of coinages were produced of precious metals ONLY. Even the first bronze coinages which represented low monetary values employed massive coins or hunks of metal. The concept of fiduciary coinage, whereby a small almost valueless piece of metal could be assigned a worth based on a governments or minting authorities say so, had not yet arrived.

As trade in currency spread, a need for smaller denominations came about. Since there was not yet fiduciary coinage, how could small change be made? After all trading was done with true metal value. The answer is that it was literally made very, very small. According to the Talmud an obol had a value of 32 prutot, based on the value of its silver content. A quarter obol (usually about 6mm in diameter and weight of about 1/8th gram!) was thus worth only 8 prutot. INDEED the coins were tiny, but they HAD to be, in order to maintain the correct value.

Of course this currency system was not practical and it did not last longer (in Judaea) than about one and a half centuries until the early 3rd Century B.C.E. when bronze coinages started gaining currency (pun intended). During that earlier time, shoppers sure had to be careful, ESPECIALLY if they had big hands.