If I'm understanding correctly what you want...
once you have selected the 'box tool', change the drop down menu to 'fixed aspect ratio' and set the height and width to the same value.
Perfect squares from then on.
The problem there is that I don't want a set size as the pictures I am using are different sizes themselves. What I want is for when you drag the corner of the box across your picture, for the box to remain a square as you move it. Then, after I have selected my area, I will scale down to the size I want. Unfortunately, if it's not an exact square, it distorts, which is a pain in the arse.
Does that make sense?