If you are building your own remember lumber comes in multiples of 4. So rather than the dimensions above think about 12x8, 16x8 you will have less lumber waste meaning less money per square ft. Always round up to how many chickens you are planning, chicken math is a real thing. Go with the biggest dimensions you can afford. If you are going with pre made coop, please do a lot of research first as many people are not happy with them and end up with added expense of modifications.