New arrivals is a great idea. There is a page (a bit hidden) for rabbits that have been listed awhile:
http://www.rabbitrehome.org.uk/longstay.asp (they are marked with red text in the results too) but no way to see newbies. I hadn't thought of that but if you are waiting for the right bun then being able to see who's been added recently would be really handy.
I'm planning on adding the search form to he side of the results so you can edit what you search for easier so they could be added into the options there.
Re sizes... well small would be littler than medium and a lot littler than large lol I can see it can be confusing though as different rescue categorize differently. Maybe if small/medium/large was defined better that would help. I'll have a think