Use a tape measure to measure the bottom half of the window in which you want your air conditioner to go. Open the bottom sash all the way up when measuring. You don't want to get the wrong size unit and find this out after you've lugged it around and heaved it up into the window. Be sure that there is an available outlet near the window to plug in the air conditioner.
Make sure the window frame and sill are sturdy enough to support the air conditioner. If the sill or other part of the window is cracked or damaged, repair it first before you put the unit in place, or choose another window. Window air conditioners are heavy. If not securely positioned in place, they can fall out of the window and potentially injure someone or damage property.
Don't use an extension cord with a window air conditioner. The unit should be plugged directly into a proper outlet.
Window air conditioners come in a standard size to fit normal-sized window frames, but their size can change a fraction depending on the BTU output of the unit; for example, the more powerful your air conditioner is, the bigger it will be.
Keep in mind that window-mounted air conditioners are meant to work most effectively with vertical-closing, double-hung windows.