I use flannel all the time and the amount of shrinkage you will experience in flannel depends on the quality of the flannel.
I have made several double-sided baby blankets using flannel, no pre washing and the flannel shrink about the same. The blankets were used for years by the recipients, from birth to preschool. I used flannel in quilts I made for my brothers and it was a more expensive flannel so did not shrink much and wore very well. Flannel makes a very soft, cozy quilt.