A breechcloth (or breechclout) is a long piece of cloth or leather (usually rectangular, but not always) which passes between the legs and is held up by a belt or waistband. The cloth itself generally about 1.75 yards/1.6 meters long, but the exact length you'll want depends on your body size, how elastic the material is, and personal preferences like flap length. As with other loincloths, you should probably make it too long in the beginning (I usually start with about 2 yards/1.8 meters of fabric), so you can trim it to the length you want and/or square the ends off.
As for how to get straight edges: Many knit materials are ridged on one or both sides, which makes for a handy cutting guide if you use a steady hand and good light. Also, most woven materials are easily torn in a straight line, especially with a small cut at one end to start the tear.