Thank you for your question. First of all, be careful how you use the word 'square'. All sides of a square have equal lengths, so rectangle is the better word to use here.
rectangle 1: 11.95mm x 6.72mm
rectangle 2: 11.2mm x 6.3mm
The first thing you need to think about is what do you mean by 'smaller'. You want to know how small one is compared to the other. There are different ways to think about what small means - we could be comparing heights, or widths, or diagonal lengths .. or lots of things!
From your question, I think you are trying to find out how much smaller the area of the second rectangle is, compared to the first. You can find the area of a rectangle by multiplying the length times the width.
After you find the two areas of the rectangles, you want to find the proportion of the bigger area that the smaller area takes up. You can find this using a ratio. For example, if the smaller area works out to be 25mm2, and the larger area 362, then the smaller area is 25/36 of the larger.
To express this as a percentage, you need express this ratio as some number out of 100.
So for this example I would have to solve for p in this equation: 25/36 = p/100.
Hope this helps,