While straw will keep the dogs fairly warm, it also attracts fleas. Better to use cedar chips. A doghouse should not be too large or too small. The idea is for the dog's body heat to keep him/her warm.

I also like the igloo type dog houses. They are insulated for comfort both winter & summer. They clean easily and last forever.

A flap on the door may not be necessary in Texas but in Michigan's sub-zero winters it's a necessity. The flaps don't seal well so there is still some circulation. At the very least, see that the door doesn't face the incoming wind.

Some of the doghouses have vents in the roof that can be opened and closed.

How cold does it get where you are?