BTU Requirements
Get a unit with a capacity as closely matched to your needs as possible. A unit that has too little cooling power will not cool your space adequately, while a unit with too much power will short cycle, which is bad for the compressor, and will not dehumidify your space well at all.

0-150 sq ft=5,000 BTU
150-250 sq ft=6,000 BTU
250-350 sq ft=8,000 BTU
350-450 sq ft=8,600 BTU
450-600 sq ft=10,000 BTU
600-750 sq ft=12,000 BTU

Installation
Window A/Cs should be installed into a window that is not in direct sunlight; if you must expose it to direct sunlight, you may want to consider building some sort of hood or awning to relieve some of the heat. Most of the A/C will be on the outside of the window. On the outside, the back of the window A/C should have at least 12" of clearance. The bottom should have at least 4" of clearance. The A/C should be tilted a bit towards the outside, so that condensate (water condensed out of the air) can drip out. The window A/C will come with an installation kit that includes support brackets for the A/C, filler kits to be installed alongside the A/C, and all necessary hardware.

Installation Questions

What is the difference between a slide-out chassis and a fixed chassis?
An A/C with a slide-out chassis has an outer cabinet that can be detached from the rest of the A/C. This can come in handy when installing the A/C into a window: instead of balancing 150 pounds of A/C while you install support brackets, you can just install the light outer shell. Once the outer shell is completely installed, simply slide rest of the A/C in place. A fixed chassis A/C has a cabinet that cannot be detached for installation.

My window A/C is making splashing noises. Is it defective or did I incorrectly install it?
Many window A/Cs make some quiet splashing noises when they are running. This is because they use the condensate water to help cool the compressor before it drains off. If this noise bothers you, check your owner's manual: you may be able to drain off the water before it is slung onto the compressor. However, this might decrease the performance of the A/C.

Do window A/Cs fit casement (crank-out) or sliding windows?
Most window A/Cs are designed to fit standard double-hung windows. For windows that slide side-to-side, there are special window A/Cs, but they are usually more expensive than the "normal" kind. Otherwise, if your window opening meets the dimensions noted by the A/Cs specifications, then the A/C should fit. Currently there are few options for A/Cs for casement (crank-out) windows. You might try a mini-split or portable A/C, although there may be some extensive installation required for either of those units.

Can't I just turn a window A/C on its side to fit my slider window?
No. Turning an A/C on its side and running it is a very good way to quickly break it.

Can a window A/C be installed through a wall?
Most window A/Cs can be installed through a wall, if the hole in the wall is sized appropriately. However, it is recommended that you check with the manufacturer and/or the owner's manual of the A/C before attempting this type of installation. When in doubt, you may want to check with a contractor to perform this service. Also please note that there are A/Cs that are specifically designed for through-the-wall use. You may want to look into one of these models for a through the wall installation.

Why are the maximum and minimum window opening dimensions different than the actual dimensions of the A/C?
Window A/Cs come with installation kits to securely hold them in your window and to fill the rest of the space on the sides that is not filled by the A/C itself. For this reason, the window dimensions that the A/C will fit vary from the actual physical dimensions of the A/C.