How much light do I need to induce a mare out of anestrus?

Unless you want to get technical, a 100 watt light bulb in a 12 x 12 boxstall is adequate. If you want a little more insurance then a 200 watt bulb should put your mind to rest. If you own stock in an electric company then by all means, use the 300 watt bulb.  Keep in mind that there is absolutely no reason whatsoever to use more than a 100 watt light bulb.  Using more light doesn't bring her out of anestrus any faster or bring her through transition more quickly.

If you want to get technical you can use a photometer.   A what?   Don't worry, you probably have one.  If you have a SLR (single lens reflex) camera put the DIN (ASA) on 400 and the shutter speed to 1/4 s.  Place a plain old white styrofoam cup over the lens. Now hold the camera as if you were going to take a picture of your mare's eye. A reading of 10 footcandles or 107 lux is sufficient to cause a photoperiod response to bring the mare out of anestrus.