We are a family of 6 (plus furry critters) in just over 1000sqft. We have 3 bedrooms (boys share one, girls the other, and everyone ends up with us by morning
). The only big issue we have is there is only 1 bathroom. If we can find a way to squeeze in another we will be set! Our basement is unfinished and usable for minimal storage (it floods occasionally) and laundry.
We find we need to have much less stuff, which I view as a plus. Our boys (7 and 5) have tall loft beds with a play area underneath. When they are a little older I will likely put desks and bookshelves under instead. They also have a common area for shared toys.
My girls (3 and 1) share a captains/mates bed. When they are a little older I will switch that out for a low loft that we will use like a bunk bed. They have a smaller room and no closet. That is the hardest. Lots of good organization furniture is essential.
Our living/dining area isn't very big either. I added some built-in shelving for our homeschool stuff and we are about to purchase a billy bookshelf system from Ikea that we hope will help contain our books and art supplies. We also use a long, skinny table which fits our space better than then the many others we have tried.
It really helps to never allow clutter to build, and to only buy things that we already have a space for. Organization is essential, because even a few things out of place leaves me tripping over toys and that makes me VERY grumpy
I have definitely learned to be a lot less messy with every child we have squeezed into this place!