Thank you so much! I found this:
Most HP Desktop PCs contain ATX power supplies that are 6 inches (15.25 cm) x 5 inches (12.7 cm) x 3.25 inches (8.25 cm). If your PC has a standard ATX power supply, you can replace it with standard ATX power supplies sold by most electronic stores.
SO does that mean I need a smaller power supply inch wise?