Is an acre of land too small

First off, I think you should consider carefully whether you want a cockerel (or rooster, if you insist on being Puritanical). Many people who've never had chickens don't realise that you don't need a male bird in the flock to get eggs. Just like every other animal, female chickens ovulate from the time they reach sexual maturity, regardless of whether they're having sex. The only reason to have a cockerel in the flock is if you want fertile eggs because you believe they're better for some woo-woo reason, or if you want to raise chickens to sell.

Cockerels can be a pain in the backside. They can be very aggressive towards people, they're noisy, and if there aren't many hens in the flock, you can have a situation where they're being almost constantly hassled and mounted by the horny cockerel.

You don't say how many chickens you're thinking of getting; that's obviously a major factor in deciding what size of enclosure you need. Chickens naturally spend their days wandering around eating greenery and scratching at the ground looking for little critters to eat. If the density of chickens in an enclosure is really high, you can end up with a bare-dirt mess, which looks horrible and isn't good for the chickens.

If you're not planning on going into egg production in a big way, but just want, say, half a dozen hens so you can have eggs for yourself and maybe to give or sell some in an informal way to family, friends and neighbours, a acre will be way more than enough. Many people in the UK keep two or three chickens in their back gardens, which look like postage stamps compared to typical American suburban backyards. If they're allowed to free-range during the day, they do rip up the lawn and scratch away in flower beds, but most people can accept that's a reasonable price to pay for the amusement value provided by the chickens.

We had a tiny flock of four hens when we lived in Italy (and also foolishly got a cockerel, which is why I'm so negative about them), and they free-ranged on our 17 acres of land. Although they could go wherever they wanted, they tended to stay fairly close to their coop, so it's not like chickens have an instinctive need to roam.

We live in the UK now, and we have friends who are sheep and cattle farmers who keep a flock of about twenty hens. I guess their enclosure is about half an acre, and they all seem very happy wandering around that and doing their amusing chicken things. They've excavated their own dust-baths, and the area nearest to their coop is pretty scraggy-looking, but a lot of the area is rough grass which the farmers have to periodically weed-whack to keep it under control.

I'd suggest that what you should be focusing on at this point is how you'd house your birds. Chickens don't really need much in terms of comfort (and I saw some really horrible, filthy, ramshackle coops when we lived in rural Italy), but you should provide a place where they can roost above the floor at night, that's protected from the worst of the weather, and where they won't be bothered by rats (any mice going for stray chicken feed in the coop tend to get eaten by the chickens, so you don't need to worry much about them).

One thing you absolutely must consider is how you're going to make the coop fox-proof. Foxes are just about everywhere, and they are a huge danger to chickens; a fox that gets into a chicken enclosure or coop will slaughter every bird in a matter of minutes. The fox's reputation for intelligence and sneakiness is justified, and you must accept that, once you have chickens, someone will need to lock them in the coop every evening. If you slip up with this or if a chicken gets left out overnight, it's virtually guaranteed that you will lose birds.

Foxes tend to hunt at night, but a really hungry fox will also hunt during the day, so you should think about that too. We were never bothered by foxes in Italy, but I'm sure that was because, along with free-range chickens, we always had at least two abandoned dogs we'd adopted, and they were free-range too. They seemed to very quickly come to see the chickens as part of their pack and were very protective of them.

If you live in a place where there are large raptors, you'll also need to think about providing at least some cover for the chickens. Even if the birds of prey aren't big enough to actually take something as large as a chicken, the instinctive response of a chicken who sees a large bird flying above her is to run for cover under a tree or bush or something. If the chickens are in a completely open area, the absence of protective cover will stress the birds.

an acre is definitely plenty of room, i use to live at a house with an acre of land and had about 25 chickens roaming around

An acre's big. And also you've got to consider the area because you need to keep them away from the neighbors' or they'll complain about the smell.

That is plenty of room

Look into poly culture and vertical farming.

An acre is perfect. A lot of people go big and get too much to manage.