Hybrids do vary quite a bit in size. I had a lovely little Columbian Blacktail who laid bigger eggs than my Speckledy, who was a very large lady, more like a good-sized Sussex. I have a real Sussex at present, who is quite a lot bigger than my two hybrids, (Amber and Silver Stars) and also my Cream Legbar, but she doesn't really take up a different amount of space in the run than the smaller birds, and they all seem to do just as much poo as each other! I think temperament has a lot to do with it. When I let mine out 'to play' after they've laid their eggs that day, they all seem quite pleased to get back in the run after a couple of hours, especially if it's a bit windy in the garden.
i think the '4 sq. metres per bird' suggestion is what foraging space is required for commercial organic freerange chickens. Obviously if you had a henhouse in the middle of a field and wanted your birds to have constant free access to grass, you'd need quite a lot of space or they'd quickly eat all the grass and turn it to mud, especially in winter. In an enclosed run like yours and mine, they do need room to move around and have sufficient personal space, so with only a very small number of chickens it's a good idea to give them a reasonably large area per bird. 3 birds could be kept in 6 sq. metres, whereas perhaps you could accomodate 6 birds in 10 sq. metres because the total open-area circulating space would be greater and you'd have more room to plan for extra perches, dustbath etc. My run is 12 sq. metres for 4 hens, and if I wanted, I think I could include 2 more, but wouldn't be really happy to keep more than that in there. At present, if I need to, I can section off the run into two spaces, to quarantine/introduce new birds or to raise growers separately, as I did this Spring, but if things were crowded to start with this wouldn't have worked so well. It also comes down to how often, and how thoroughly, you are going to be prepared to poo pick after them, as with a greater concentration of birds in a small space, things get very dirty fast, because they tread and scratch in the droppings before you can get to them to pick them up. Whether your run is roofed is important as well - a dry run will keep cleaner than one exposed to heavy rain.
i do think 3 is a good number to start with, because a group of 3 somehow looks much nicer, and also, if and when one dies, you're not left with one lonely chicken. However, at this point, you then need to have room for 2 new ones, not another single one, as it's much harder to introduce a single newbie than a pair. So again, starting with plenty of space in relation to your total flock is a good idea. We all know how chickenitis takes us over! Anyway, good luck, i'm sure they'll be fine.
