I started measuring "before and after" a couple of years ago to see if it was really worth the effort of washing the fabrics before cutting them - and if I was imagining the shrinkage or if it was really occurring.
Some cottons don't seem to shrink at all.
Some shrink "a lot" - like two inches or more in width or length or both!
I bought a length of Roc-Lon tea dyed muslin that said it was pre-shrunk. In my opinion, that was an inaccurate statement. It shrank a LOT more after I was done with it!
I made some "pillowcases" for some couch cushions - one of the very few times I did not wash the fabric before cutting it - they were "baggy" when I made them. Now I need to wrestle them off and on the cushions. This particular fabric has always been washed in cool to tepid water - dried to damp dry - and put back on the cushions while damp just so i can get them back on.