For how many chickens? Will you need a baby pen in it, and a grow-out pen as well as general layer flock area? How many roosters, if any? Isolation/jail/roo/broody pen needed? For bantams only, large only? How large?
What kind of weather/climate? Need four walls or can two be open wire?
Tall enough to walk into, or a half-high one up on 3 or 4' legs, and reached into to clean, etc?
How much money do you have to put into it?
I generally go look at my scrap wood pile and see what I have on hand for 2x4s, plywood, windows, wire, etc.
Then figure what I'll have to buy new and what I can scrounge. Check Craigs List.
To keep costs down, I start with figuring the minimum number of sheets of plywood needed. One sheet for the floor makes a coop 4' x 8'. Two sheets for the floor makes a coop 8' x 8'. Three sheets for the floor makes a coop 8' x 12'. I've found four sheets for the floor makes the most usuable size for me of 8' x 16' - good for a flock of 15 over a long winter with deep snow when they won't go outside for 5 or 6 months.
How high? An 8' tall wall, count how many sheets of plywood standing up all the way 'round.
A 4' outside wall on two ends, with a center peak of 6' to 8' saves plywood on the two ends.
Takes more plywood for roof than the floor to allow for an overhang all the way round - 2' overhang is nice although this is your weather/sun conditions and personal taste. One side of the roof can go out 12 feet or so to make a covered run - nice to prevent mud in rainy weather.
My first coop was a wooden shipping crate with the open side covered with chicken wire, and a little door for me to get in. The chickens and loose rabbits loved it. The wooden shipping crates are 8' square floor and 4' tall (laying on its side).