iTunes 12: Is there a way to increase album art size in the Album View
I have the same question but no solution.

Someone somewhere advised an app: Swinian. It has the slider for changing the artwork. Very nice app. one month free trial.

Other (obvious)workaround is changing the screen resolution. I use an app Switchres.Easy and fast but ofcourse it changes the resolution everywhere and not just iTunes album view.

It works but not ideal or perfect.