Buy an extra set of cases with which to do a test. Measure both length and width very carefully before and after washing and drying on the hottest most shrink-inducing cycles you can muster. Repeat the wash and dry test a few times as some cottons will continue to shrink over several cycles.

That will tell you more about the risks of that particular fabric. Some flannels are pre-washed and pre-shrunk before or even after being sewed. Hot water and drying with all-cotton items is always a possible source of shrinkage.

I wash my flannel sheets (LL Bean, Land's End and Garnet Hill) at 150-190F. I run them through about 15 min. of a no-heat dryer and then line dry them outdoors. I haven't experienced any noticeable shrinkage, but one never knows. (And I don't happen to have any fitted flannel sheets which is where shrinkage would be the most critical.)

HTH,

L.