Your size problem may be related to cutting, or sewing, or pressing and possibly a combination. One of the quilting gurus (can't remember who now) gives a very detailed account of creating the perfect block.
1. Measure each piece before sewing to make sure it is correct.
2. Sew, carefully press and measure the sewed piece to see if it is the dimension it needs to be. If it is not, then check where you go off. (Is the seam to wide or is the pressing wonky?)
As for the thread: I like the variegated threads as well.