We get Rosie chickens in California which typically are 4-5 lbs. No problem at all with Marcella's lemon recipe nor with any other favorite recipes. But I'd absolutely recommend investing in an instant-read thermometer to make sure you get a 175-180 reading on the middle of a thigh without it touching the bone before removing from the heat and allowing to rest for 10-15 minutes. Instant-read is an indispensable kitchen tool IMO.
The farm where I get my chickens always just has 5-6 lbs, they seem to work fine in most recipes and I just cook until the temperature on my thermometer says they're done! I've never used that particular recipe though. I usually brine my roast chickens and they turn out just as moist as the little ones.