Writing in around A.D. 204, Tertullian explained that Christians mark their foreheads with the sign of the cross.
In all our travels and movements in all our coming in and going out, in putting of our shoes, at the bath, at the table, in lighting our candles, in lying down, in sitting down, whatever employment occupieth us, we mark our foreheads with the sign of the cross. (Tertullian, De corona milites, 3)As is obvious, the original sign of the cross was not the large one that we make from our foreheads to our torsos and then across our shoulders. St. Cyril of Jerusalem confirms the same:
Let us then not be ashamed to confess He who was crucified. Be the cross our seal, made with boldness by our fingers on our brow and in everything; over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are traveling, and when we are at rest. (Cyril of Jerusalem, Catecheses, 13)The widespread use of signing the forehead with the cross is likely Apostolic in origin. I believe that Revelation 7:3, 9:4, and 14:1 are referring to the practice. These redeemed men are "signed on the forehead". Of course, the imagery comes from Ezekiel 9:4 where the faithful are sealed upon their foreheads with a mark of redemption.
In his commentary on Ezekiel, Origen interprets the meaning of the mark:
“The shape of the letter ‘Tau’ presented a resemblance to the figure of the Cross and that therein was contained a prophesy of the sign which is made by Christians upon their foreheads, for all the faithful make this sign in commencing any undertaking and especially at the beginning of prayer or of reading Holy Scripture. (Origen, In Ezekiel, 3)As to the origin of the large cross that we make today across our shoulders, does anyone know when it became popular?