I meant that the 250 watts in that custom pc build are just to power the components it is sold with, not meant to add more stuff. I can tell that the system will make use of 150~200 watts and the GPU is rated with a tdp of 55 watts so you will be cutting it close.
About the size, it isnt that "huge", worst case scenario it will fit directly in front of the drive bays of that case.