I'll tell you using a length of 8 1/2" for each side. You use the same method for other sizes.
Take a sheet of 8 1/2" x 11" paper. Fold it in half and crisp the fold so you've got two 4 1/4" x 11" rectangles. Unfold. Bring one corner up to the center fold line. Trace the 8 1/2" edge of the paper from the point on the center fold down to the corner. Unfold. Bring the corner that you just drew into up to the center fold line. It should it at the same point as before. Again trace the 8 1/2" edge of the paper from the point on the center fold line down into the corner of the paper. You've now got an 8 1/2" equilateral triangle. You can do this with any size paper. Just start out with a square that has the same length as the length of the triangle edge.