how to measure in gimp?

[rosskane]

i have wide feet, the only wide that fits me is new balance extra wide. but recently, i have noticed new balance extra wide doesn't fit me. what i am trying to figure out is if they made their shoes more narrow, or if i have some kind of a medical condition causing my feet to get bigger. in the picture below, I am trying to compare the width of the new shoes with the old ones. but i dont know how to do it in gimp. i know the measure tool exists, but i dont know how to point it in the right spot, and i dont know if i am drawing a straight line.