This is my thinking. Three unfinished 2 1/2” squares lengthwise x three unfinished squares widthwise will equal 7 1/2” unfinished square. You will have 2 seams in the center both lengthwise and widthwise so if you make the seams 1/2”, you will have 2” less when the center part is joined. The 5 1/2” square will not butt up to the 5” square. Do you have a rotary cutter, mat and template? Sorry, but I’d recut the small squares. Welcome to the board. You will learn so much here.