Hi Barbrose, practice sewing a 1/4" seam by cutting three 1-1/2" strips 10" long. Sew the three together and measure the center strip in width. It should measure 1" exactly. If it doesn't, do it again, and again, until you can get that 1" exact measurement in the center strip every time. Practice does make perfect!
Also use the same brand rulers for cutting your pieces and place edges in the center of the line on your rulers.
I think these are the two basic reasons why your blocks would be different sizes.
Ironing the blocks too vigorously (press the blocks) can also lead to stretched blocks.
You know it doesn't matter much what seam allowance you make (except it changes the size of the finished block) as long as you make all seam allowance the same size.
Good luck on this...you will get it!