You cannot do 100% crops in Lightrrom, you can choose an aspect ratio and crop any area of your image, but you don't know how many pixels that is, on Export, Lightroom will re-sample to whatever you specify.
So taking one step back, 100% crop means on your monitor at native resolution one image pixel is showing on one monitor pixel. So different screen resolutions will show that 100% crop at different physical sizes, with some Retina displays the graphics chip will re-sample the image anyway!
So, if you have a 5D mkIII you have 5760px x 3840px, the forum takes 700px long edge images inline. So if you want a 100% crop you need to specify a 700px x 700px crop as in PS or a similar program, not a 1:1 crop and then exported at 700px as in Lightroom.
100% crops are relevant if you realise everybody will see them at a different size, it is showing the pixel level detail of your images, regardless of the viewers screen.