You are asking for a feature similar to the one that I and another select group of fine individuals have been asking for for a short while--and I've been asking myself for months.
If I understand correctly, you want to just select songs and have Media Center resize the cover art and save it out.
I know people who have re-encoded their entire MP3 library just so it would be all the same. Most people know that adds a generation and, I think, even when going from greater to lower bitrates introduce horrible artifacts that decrease the audio quality overall. I'm sorry to say I did that myself years back when I thought 64 Kb was enough. Then I got good speakers--lol. Woops.
I'm not sure why you would want to resize your cover art to all be the same? If you have a specific reason, it may be a good one. I personally DO have all my cover art (except one, lol) saved at exactly 1200x1200 pixels. But, the only reason I do is because I scanned everything in by hand. I decided that the extra requirements of having FLAC warranted a little extra work on my part to get gorgeous cover art. If you're downloading 500x500 pixel cover art from Amazon, I see no reason to resize to 300x300.
The request many of us are making is for an on-the-fly resizer for handheld devices. That has a valid purpose for saving space--sometimes not much but in my case: 1200 -> 320 pixels can save a ton of space.
I'm thinking that our handheld version of this feature request may potentially be able to handle both options...I'm very hopeful that my request will be added!