It was 1200 x 1200 pixels, but recently changed to 1400 x 1400 pixels:

To be considered for promotion on the iTunes Store, your podcast cover art must be 1400 x 1400 pixels or larger, in .jpg or .png format. Please note that all images must be formatted for RGB color space. CMYK is not supported.

iTunes now supports 1080p HD video, presumably to better support the recently announced Apple TVs that also support 1080p HD. And Apple is now doing more indexing of episode metadata:

We now index episode metadata in search. Include descriptive episode titles and summaries for best Search Engine Optimization on iTunes.

Something else that's new (to me) is including different artwork for each podcast episode:

We encourage providers to include episode-specific cover art at the episode level. Further details can be found in our online technical specification.

A bit more work to put out each episode, but most podcast producers are likely editing artwork for their episode posts on the web what's one more .JPG?

Full detail can be found at Apple.com's Podcasting specs page.

Why the Change?

These changes and improvements to the podcast spec were put out a few weeks ago and lots of rumour sites grabbed onto them as proof of a TV from Apple. In hindsight, it looks like it might have just been to help podcasts look better on the newly announced MacBook Pro with retina display.

Not that a TV isn't coming sometime, but just not right away.