The coop can start to stink like ammonia if it is not cleaned aften enough. the poo piles up under the roosts because they digest and poop all night long. If you keep that cleaned out there is no smell. The actual birds have a mild bird smell, but you only smell them when you are holding them enjoying their birdness!
There are as many different techniques for keeping the coop clean as there are hen keepers. I put pine shavings in the coop and stir the pile under the roost avery day or two. When it gets icky, I throw some more shavings in until it is too big a pile or gets any odor at all, then I shovel the whole pile into the compost pile and put new shavings in. This is about every three weeks, because I want the compost and like the coop to be fresh. Since the mess is mostly still wood shavings it is lightweight; moving it around is no big deal.
The size of the coop should be larger than your current plans, because you WILL end up with more chickens than you ever thought you'd need!