The size of the hole depends on the size of the bird you want as a tenant....small for Wrens, larger for Sparrows, etc.
I soak the gourd first in bleach water then scrub off the skin, drill several drainage holes in the bottom, cut the entrance hole and coat with exterior urethane.

Here is a link that might be useful: {{gwi:871056}}

Sue-The size of the "door" hole depends on the kind of bird you want to attract.I usually drill a 1 1/4 to 1 1/2 inch hole in the ones I sell.This will attract the smaller birds,like wrens.You also need to drill 4 or 5 tiny holes in the bottom for drainage.If the gourds are completely dry,like wood,they shouldn't rot.I paint on the gourd houses with acrylics so I always put 3 coats of an acrylic sealer on them in case they are used outdoors.If you are leaving them plain that wouldn't be necessary,but it would bring out the beauty of the gourd.
The birds will nest in the gourds if they are hanging from a tree,or just about anywhere else.I drill 2 holes in the top and put wire or cording through it to hang with.Good luck............Lillie

Here is a link that might be useful: My Gourds

Concerning the entrance opening. There are lists available listing the size of opening required for each species of bird you want to nest, in your house. Should be easy to find on the web or bookstore or bird supply house.

I cut my openings with a hole saw, which you chuck into an electric drill. You can buy them in various sizes. It is a drill bit with a standard drill bit in the center to guide your hole, and a much larger surrounding bit with jagged teeth arranged in a circle.

You have to be careful and go slow, as these bits can grab and tear up your gourd. But once you get the hang of it, you can cut a perfect circle of whatever diameter you choose, very quickly. Home Depot or hardware store keeps these in stock.

As for finishes for outdoor gourds, I have found that all the clear finishes that I have tried [ polyeurathanes, linseed oil, etc. ) either peel off rather quickly or evaporate rather quickly, in about a month or so. I am sure the paints, as posted earlier, would last much longer.

An unfinished gourd birdhouse or feeder will last about 2 years if left outside with no protection from sun or rain, but they are easy enough to grow and replace.

Good Luck