Birdhouse "Marigold" - Wood Nesting Box - Sneak-A-Peek Window

I just received this bluebird house ordered through Walmart last week. I am very happy with the quality and construction of this birdhouse and it is made in the USA, which is a plus. My concern, though, is that the floor space of this house is smaller than it seems from the dimensions listed on the description. The floor space of my birdhouse is about 3.25x3.75 inches, which is not very much space for an average nesting of bluebirds. It is my understanding that bluebird houses should have a floor dimension of 5x5. I am worried the space inside this birdhouse will be too small, especially in the heat of the summer when baby birds are cramped inside. That being said, there are plenty of wrens and chickadees in my yard who probably won't mind tighter quarters, so if the bluebirds decide not to nest in it, I am sure it won't stay vacant for long.