A Simple 2*2 doors can be opened by stepping on the stone pressure plates. You can add stone pressure plates on both sides of the door.
This guide will let you know how to make a simple 2*2 doors. Follow the instructions in order to make one.
You will start from placing two pairs of pistons on the ground and leave a space of 4 blocks between them.
The next you need to do is to place a gray concrete block on the outer side of pistons and use redstone at the top of these concrete blocks.
Start digging the ground from the left concrete block up to the right one. And then add a redstone torch under both gray concrete blocks. Torch will push the pistons.
Put some redstone dust on the ground you just dug. Then you need to cover the ground and add a stone pressure plate in the middle area as shown in the image below.
Now you need to connect the opposite side. Dig the ground and use redstone durt under it to connect. And then cover the ground again with white concrete. Step on the stone plate to open and step back to close the door.