The issue of overpopulation isn't that there isn't enough space for people to live in.
If you gave every person in the world 2500 square feet of land, they could fit in the state of Montana.
The issue here is that you can't feed all these people and create enough power for them. Not to mention the issue of job competition, disease, clean water, pollution, etc.
7 billion times 2,500 square feet = 17500000000000 square feet, aka 3314393939 square miles.
There are only 3794083 square miles in Montana.
You'd need nearly 900 Montanas to fit them all.
EDIT:
Actually, you're right.
I just finished calculating it and you could only give each person maybe 582 square feet.
Point is, land still isn't the issue.
Oops, you already did the math too.
Still, my brain just wouldn't let that gross inaccuracy slip past me. I should have checked posts before typing it.
EDIT 2:
Wait, even giving them 582 would still require 200 Montanas.
Though, the world actually can fit 200 Montanas in it. Montana can't.