Please find a patch that replace thumbnail by the resize picture below 450px in the content of the rss feed generated by Gallery2.
Result can be seen here : http://www.xeberon.net/gaetan/gallery2/rss/Portfolio (rss)
and here : http://www.xeberon.net/gaetan/dotclear2/static/Photoblog (the reason why I've made these changes)
Please review the code and tell me if I've done everything wrong. If you intend of merging this featurein the baseline, I'll take care of cleaning and add the checkbox and the maxsize in the admin page.