@bellaisa many folks on here will recommend 4 square feet of coop and one foot of roost per LF. Sometimes you may get away with less, if you have bantams, if your birds free range, if your birds are not locked up at night or in poor weather and have other options to roost or hang out. You can always try with your existing setup before jumping into buying or constructing a second coop. Watch for signs of stress beyond the new pecking order being established with the introduction of the new birds. Your concerns about winter are valid. What may work in terms of space now, may no longer work once they spend more time indoors over winter. You don't want to be doing emergency coop construction in the snow. You could start a new thread with pics of your existing coop and ask for advice on how it could be modified to better accommodate more birds. Perhaps an addition would be easier than all new. Your chickens are likely used to the space they have now and by adding 6 new birds at one time, I suspect they are going to notice and feel it cramps their style. The pre-fab coop you posted does look better than some others I've seen. When you go back to look again, check to see if there is perhaps venting other than the windows, up along the roof, perhaps under the eaves. Also measure the floor space of it excluding the nest boxes.