In the name of the Holy, Blessed, and Glorious Trinity, proceed we to our work in these mysteries to accomplish that which we desire; we therefore, in the names aforesaid, consecrate this piece of ground for our defense, so that no spirit whatsoever shall be able to break these boundaries, neither be able to cause injury nor detriment to any of us here assembled: but that they may be compelled to stand before this Circle, and answer truly our demands, so far as it pleaseth Him who liveth for ever and ever; and who says, I am the Alpha and Omega, the Beginning and the End, which is, and which was and which is to come, the Almighty; I am the First and the Last, who am living and was dead; and behold I live for ever and ever; and I have the keys of Death and Hell. Bless O Lord! this creature of earth wherein we stand; confirm O God! Thy strength in us, so that neither the adversary nor any evil thing may cause us to fail, through the merits of Jesus Christ. Amen.
It should be made traditionally 9 feet in diameter though as long as it's big enough to faint safely with rolling or falling out, your fine. It can be drawn on the ground with blessed chalk, Enscribed with a consecrated knife or sword, or painted on a large piece of cloth and rolled out every time you need it. You can buy these types online but really, it's better to make your own. Some people have a room which they dedicate completely to Solomonic magic called the oratory or temple. Sometimes, they will paint a permanent circle. This can
Cause trouble because some rites require the circle to be in different directions.
When evoking a powerful spirit, they will to all sorts of things to scare or tept you out of your circle. Think of your circle as your fortress. Have faith in it. If you make it right, no spirit will cross. Also, here's a story I found in a grimoire
Instances are recorded of many who perished by this means; particularly Chiancungi, the famous Egyptian fortune-teller, who in the last century was so famous in England. He undertook for a wager, to raise up the spirit Bokim, and, having described the circle, he seated his sister Napala by him as his associate. After frequently repeating the forms of exorcism, and calling upon the spirit to appear, and nothing as yet answering his demand, they grew impatient of the business, and quitted the circle, but it cost them their lives; for they were instantaneously seized and crushed to death, by that internal spirit, who happened not to be sufficiently constrained till that moment, to manifest himself to human eyes. --