Smaller Album artwork in iTunes 12.5.1.21?
I don't think there is anyway you can make them smaller.  The Grid view, in Album View, I think is inoperable to size change.  For me, I like larger album covers so I deleted the microphones in Artist view (and Artist view itself) and use List in the Album View.  Of course, I can expand to about 14" x 14".  The only smaller album coverwork are the microphones but they seem to be controlled by the iTunes Store box sets, so many people have missing artwork---and lots of microphones.  That's why I eliminated them.