Or - why your block is not finishing at "the intended size"
There will be absolutely nothing in this post that has not been mentioned many times before - but there might be something that a "newbie" has not thought of yet.
1) Some unwashed fabrics shrink when they are steam pressed. This will alter the finished size.
2) If one is not using a die-cutter for cutting the pieces, there may be variances in the size of the piece from the "exact" size. The way I cut squares - for example - I know they are about +1/32 of an inch from the "true" size. That does make a difference in a block with many pieces.
3) The seam allowance - what really matters is what is "showing" between the seam allowances. Doing the three strip test to check before starting out is worth the time and effort.
Here is one reference for how to do that:
http://www.connectingthreads.com/tut...ance__D95.html
4) If one uses starch - don't press/iron a soggy wet piece like you are rolling out pie dough. It will stretch the fabric.
5) Flatten rumpled fabric before cutting it. Iron or press it - I think using steam is better than a dry iron, but that's my preference.
6) I've heard that thread thickness matters. I have a huge supply of Coats & Clark cotton covered dual duty thread that I want to use up. Lots of threads about preferred threads on this forum.
7) Make a test block to see if it is turning out the way you want it to. Then make the adjustments. (of course, knowing where and how to make the adjustments comes from experience!)