UPDATED: Re-Apply Downsized Artwork v3.0
Re-Apply Downsized Artwork v3.0 will examine the first artwork of each selected iTunes track (or tracks in the selected playlist) and if greater than a user-set resolution (n x n pixels) will downscale the image to that resolution. Artwork already at the set resolution or smaller will not be affected.
This latest version has substantial UI and feature additions: cycle through each selected track; displays track and artwork information; preview the artwork at the selected resolution; pad the image to make square; can insert as track's first artwork to preserve current artwork(s) instead of replacing the first artwork; apply settings to all selected tracks or just the current displayed track; view image at actual pixel dimensions or proportionally sized to the re-sized window; export the displayed image as a file.
Re-Apply Downsized Artwork is $1.99 and free to try full-featured for ten days. OS X 10.8 or later only. More information and download is here.