What birds are you trying to attract? Some require different sizes.

The link below lists the hole sizes and other dimensions to be considered as well.

Will you share a pic with us once it is completed? hint, hint...Pleeze....

Sue

Here is a link that might be useful: Building Bird Houses For Wrens, Bluebirds and More

Thanks so much for the information. I have a lot of small birds in my yard I am unsure what they are(wrens maybe) but as I said big birds have plucked the babies out of the houses in the past because the openings were too large. I am going to make the opening large enough for wrens and rework some of the older houses as well.
K.