Evaluating the crush

A good grain grist will contain an assortment of very coarse to very fine grain particles.
Some amount of very fine ground grain, in the form of flour, will not be a problem, but should represent a low percentage of the total grist. Too much flour-fine grist can lead to a stuck sparge, driven by a doughy mash. The goal of the crush is to crack the grains and expose the starches within. A finer crush will increase this at the expense of removing the husks. Another penalty for too fine a crush will be residual grain particles getting through your natural "grain bed" filter and ending up in the wort.
A medium crush will avoid these problems and will have comparable mashing efficiency.
A very coarse crush can lead to lower mashing efficiency.