There are several things that can contribute to an incorrect size of a finished block. First, make sure you have cut the pieced correctly. Sally Collins has an EXCELLENT book titled "Mastering Precision Piecing" that will help. When setting my ruler down to cut a piece I always make sure to use the full measurement of the line on the ruler (meaning the fabric covers the whole line, not the left edge of the line on the ruler, but includes the right edge of the line on the ruler). In addition to the cutting is sewing and pressing the pieces. I always find a scant 1/4" works best. Be careful not to stretch the pieces when pressing and always press each piecing step before going to the next. If triangles are not cut and sewn properly, they too can affect the size and squareness of the block. Finally, the more fabric pieces in a block, the more chance for distortion in the final size of the block. Good luck and hope this helps.