[Kaihan]

For me, my method would be to use "Canvas Size" under Image to choose sig size.
Then select all the Layers and press Ctrl+T (Transform) which would let me move and scale all the images.

This is of course after all the layers is in Photoshop already.

Edit: Forgot to add, once in transform mode, hold shift and select the corner of the box if you want the image to be scaled evenly without stretching.