Hi,
Well done for thinking of rescuing some bunnies! Would standard rexes be big enough? I have some needing homes, they should grow to be on the larger side of medium, the mum needs a home too, see here:
http://forums.rabbitrehome.org.uk/showthread.php?t=291349
eta- they aren't neutered but I could have them done before they left, or you could do so yourself (if they are a same-sex pair they wont need separating for neutering)
How big is the covered section, the 'hutch' bit of the chicken coop? Do you know how wide the coop is? The reason I ask is that most rescues will look for a certain amount of space covered, and a certain amount overall for the bunnies to run about in, so you may have to modify the coop a bit.
Most rescues will have rehoming requirements roughly based on the Rabbit Welfare Association's guidlines, although all are individual, so have a look, you can find them here
http://www.rabbitwelfare.co.uk/ahutchisnotenough.htm