This is a two way hidden door. It uses a pressure plate on each side going each way and closes as you pass. This is the smallest i can figure to make for this style, 4 blocks tall from feet, 3 blocks under your feet, 7 blocks wide, 6 blocks thick. It uses 6 sticky pistons.
The assembly will appear with you to the north of it standing at the entry point. Change the stone blocks to blocks of your choice.
If anyone has a better or smaller i'd like to see it, but this is what i came up with for this style. thanks.