For freestanding greenhouses, upper vents should equal 20% of your floor space. Lower ventillation should equal 10% of your floor space. In an attached greenhouse, total venting area should be equal to 25% of your total glazing, or wall space. You can count your open door as ventillation. Try to make your upper vents larger than your lower vents.
It is more effective, and productive to use an electric fan, as opposed to using a box fan on the floor, to blow around hot air. If you use an electric fan, place it high. It will pull in cooler air, using your lower vents, and blow the hot air out. You can use your electric fan, in conjunction with a thermostat. All you do is set the target temperature. When the internal temperature rises above your target, the fan will come on, lowering the temperature.
A very good way to prevent insects in your greenhouse is to use a screen, over your vents. You can buy official 'Greenhouse Screening Material', or you can visit your local hardware store, and see what is available. - landscape fabric, for example. You can also use cheesecloth.
Misting for Cooling
As water evaporates, it cools the air around it. This is why misting and cooling systems work so well. (If you live in a very humid climate this will not be as effective.) Just wetting the floor of the greenhouse will offer some benefits, on its own. A fine mist near an air intake vent increases effictiveness. Make sure that the system you choose produces small dropletts, so you don't have puddless in your greenhouse.
Foggers work on the same principles. They use even less water, and are typically installed in the ceiling. They are especially useful for seedlings.
Evaporative coolers operate by forcing air through a moist fibrous material. They use significantly less resources. They are used mostly in very dry areas, but can be utilized by anyone. They are also known as swamp coolers.
AC or Refrigerant Cooling
You may consider extending your home's AC to your greenhouse. This can be cost prohibitive, for most, however. The optimal candidate for this would be the lean to greenhouse.
A shade cloth is applied over the top of the greenhouse. On a very basic level, you are simply creating a barrier between the sun, and your greenhouse. If you use that principle, you can get fairly creative, in design. For example, if your greenhouse is not exactly a gardener's delight to look upon, consider covering the ouside with muddy water. You can also use glazing treatments, to do the same job. They just have to be painted, or sprayed on.