The size obviously is to your own needs and requirements A good size for the average small garden is eight feet x six feet.

My aviary was made using a wooden garden shed of that size, it has an apex roof. I insulated the entire interior using plywood, it has a wooden floor which I have covered with [rhino floor covering] this is easy to keep clean.
On top of this I put sand. I have portioned the inside of the shed and fitted another door at the far end which leads out to the outside flight.
The outside flight runs alongside and is the same dimensions as the shed.
The roof of the flight is shaped similar to the roof of a Victorian conservatory and is more pleasing to the eye than the usual flat roofed flight. [See photo].
The floor of the flight is concrete, the wooden sections are 1 ½ inch planed wood.
The mesh is 1x1/2 inch stapled to the wood from the inside.

I made the mistake of fixing the mesh to the outside of the roof to start with, so the love birds in particular were able to get at the roof beams and chew away at them at their leisure.
The flight also has brick footings 6 inches high.
The inside flight also has a section which is fitted with mesh of a size which allows only the finches to get through this allows them to nest undisturbed from the love birds [who sometimes can be bullies].
All the birds have access to the outside flight at all times.

The inside areas are fitted with tube heaters on a thermostat, they only operate in cold weather.

I also keep two breeding cages in the shed section of the aviary. These I use for any new arrivals and as a hospital cage if the need arises.

I have tried various plants and shrubs in the flight but I am afraid that my lovebirds take a delight in destroying them in a matter of hours