You should get through your first winter with the current flock and then make adjustments next spring (new coop, chick additions)
Last year was my first winter with my 3 chickens. My coop is 4X4, run was 44sqf. Believe me, when weather keeps them in, that space was really to small. I didn't wrap the run, I just covered it so snow came in the sides.
This year I extended the run a bit, it is now to 64sqf and will wrap the entire thing with clear heavy duty plastic and vent the top sides. I'm also in the process of building an 8X4' coop, not including nest boxes. I think the extra space will be appreciated.
Just my 2cents...