Wrens prefer a house that has an entry hole dimension of 1 1/8 inches, any larger and house sparrows and other less desirable birds will use them.
The floor should be 4 inches square and 3-6 inches below the entry hole.
Do not add or buy any birdhouse with a perch. The birds don't need it, whereas house sparrows and other less desireable birds will have easy access if perches are used.
These small birds require small houses; they only need enough size for their nest and babies. The bottom floor of the house for both the Carolina wren and the house wren should measure 4-by-4 inches. The height of the sides should measure 8 inches tall angled down to 6 inches. It doesn't matter whether the top angles toward the front or toward the back; this will depend on where you place the box. This allows for a sloping roof that will help shed rain water.
The size of the entrance hole is one of the most important factors when it comes to designing a birdhouse for a native species. If the entrance is too big, invasive species or larger native birds can quickly steal the home from the wrens. House wrens require a hole with diameter of 1 1/8 inches to 1 1/4 inches. The Carolina wren requires a slightly bigger hole -- a diameter of 1 1/2 inches. Drill the hole approximately 4 to 6 inches above the floor.