I go by the sizes of the "toothbenders" sold by the Birdwatchers General Store on Cape Cod (a place I visit whenever I am there). They sell toothbenders for chickadee nestboxes at 1-1/8".

Is this all the advice I'm going to get? I asked more questions than just those concerning the hole reducer size. Some people say that chickadees are not tolerant to monitoring and that the hole reducer should be installed only if there an active nest. I need to know what I should do to not spook her.
She is not incubating yet, but she sleeps on the nest. There are three eggs so far. I installed a camera in there (it works only from 6PM to morning right now because she is not there in the daytame) Link is below. I'm not sure if she will have success with this since we don't get lot of help here.

Here is a link that might be useful: Chickadee nest webcam

Well I am a little confused as to what more help you need. Since you already know she has eggs you can add a 1 1/8 reducer as having eggs means it is an active nest. Since you have a inbox camera you shouldn't need to open the box anymore but why does it only work from 6PM to morning, why wouldn't you be able to see with the camera when she isn't there? What other advice are you looking for? If you have House wrens you could add a wren guard as they can get into a 1 1/8 hole. Sialis.org has info on the wren guards I used one last year on the house the chickadees chose as we do have House wrens. I monitored our chickadee nest about every other day to every third day until the babies were old enough to fledge early then I just waited and watched from outside. Would love to have a inbox camera so I wouldn't need to open nestboxes to check on them.

Donna

I thought I posted but dont see my post. My Dees are fine with me monitoring, but they are here year round, they know me and trust me. They are always within inches of me when I fill the feders, even had a few eat from my hand in the winter!

Donna, I just installed this camera two days ago and when I did, I didn't know that working around birdhouse would spook her. Some birds could abandon even an active nest if you scare them. I also don't know if I should wait for her to start incubating before installing the reducer. Bluebirds could come to claim this box (this is the box they always use), but that is not usually until late summer.
Camera is off because there is nothing to see there in the daytime, but I can turn it on any time I want if I need to check it out.

Oops. I mistakenly I logged under my old nick (aleksandras), but it is still me, the original poster :)