I'm a newbie, and many generous chicken keepers may not agree with my numbers. But I've got a coop similar to this, and have 2 barnevelders, 2 RIR, and 1 shaver (a NZ egg layer)...
BUT, my birds have an enormous pseudo free-range environment. They're behind a fence with plants to dust under, 2 waterers a chookateria for food, and they're allowed in/out all day everyday. The fence line has a ridge of blocks (coincidence) and the birds were out all day everyday, even when it snowed last year (1ft snow is a rare occurrence here)
I've tinkered with the coop inside (new staggered roosts and ramp indoors) to allow the ladies space on each roost and they have 1.5ft of roost each.
So if you can only manage a small coop, you need to have ranging birds. All my girls are happy, and get out and about scratching all day.
Eventually I will build a bigger coop, and get more birds, but again, they'll be free range.
And FYI, I live right in the middle of town, so these are urban birds.
		
		
	 
I'm not arguing with you at all.  It may sound like it, but I'm really not arguing, just trying to explain my thought process.  If you can provide lots of space outside on a regular basis and about all you use your coop for is a place to sleep and maybe lay eggs, you need very little coop space.  But you have to make a commitment to not leave them locked in the coop much at all during their waking hours.  That's assuming you lock them in the coop at night.  Some people don't.  People get hung up on coop size.  What matters is space available, wherever that is and how it is provided.  
This does not only mean getting up in the morning to let them out, but allowing them to choose when they go to bed.  When I have immature chickens in with the flock, some older ones are sometimes pretty vicious toward them on the roost until it gets dark.  So my immature ones don't hop up onto the roost until it is getting pretty dark.  My coop is big enough that I can lock them up early and they'll stay on the coop floor as long as they can.  In a smaller coop that could be a problem.  Mine is big enough and laid out in a way that I have some flexibility in how I manage them.  I don't necessarily let them out first thing in the morning either.
It depends on the flock make-up too.  I hatch chicks every year in an incubator and when a hen goes broody, she gets to hatch and raise chicks with the flock.  If space were tight, I could not do that the way I do it.  Have you planned how you will integrate new chickens in a few years?  From your post, I assume yoou will build a bigger coop, but that may be something to consider.  
I realize in true urban/suburban areas you probably don't have a huge risk from daytime predators, though I'm not that familiar with New Zealand wildlife.  If your yard is fenced where dogs can't get in, your biggest risk is probably a hawk or something like that in the daytime and your hawk risk is probably not that great.  In urban/suburban areas, dogs can be a huge risk, day or night.  But what would you do if your chickens started getting killed off?  What if every other day you found one dead and partially eaten during daytime?  What options do you have if your set-up requires them to free range?  When I lose one to a predator, I can leave them locked up in the coop and run for weeks without a problem.  
The first time my chickens see snow, they stay away from it.  But if it stays on the ground for a few days, they will go out and forage in it.  They get used to it and the cold does not bother them.  But I find that mine really hate a cold wind.  They'll go out in 0* Fahrenheit weather and forage in the snow, but if a cold wind is hitting them, they find a place the wind is not blowing.  With mine, that is either in the coop, in a shed if they can get to it, or a corner created by the gate and the coop.  If yours are out, I imagine they have decent wind protection.  And yours may be more willing to explore in snow because it is so claustrophobic in the coop.  But that is just supposition.  I'm not a trained chicken psychologist.
Each if us are different and with our own unique circumstances.  There are a tremendous amount of different circumstances and different management techniques to deal with those.  I really don't like hard and fast numbers for anything to do with chickens since there are so many different circumstances.  And I don't recommend providing as much space as you reasonably can for your chickens because I am worried about their mental well-being.  I find it makes it easier on me.   I try to plan for the worst days when things go wrong, not the good days when things go right.
I really hope you continue to be successful in what you are doing.  I don't have a problem with your numbers for you in your circumstances.  There are a lot of people that do it a whole lot like that.  But there are a lot of us that can't do that.