Hello @Jockipocky
I assume you are wanting to upgrade the GPU to a 2080 as per your other post.
I would go at least 650w or higher. If you find a better deal on a stronger PSU, take it.
It holds a standard ATX PSU. Make sure the new PSU will have the 6 or 8 pin requirement for the card.
the width and height will be standard. You can measure the depth with a tap measure to make sure you will have enough room for the new PSU, but you should be ok with just about any one you choose.
Here is a video on how to replace the power supply...
http://supportvideos.ext.hp.com/detail/video/5106203213001/removing-and-replacing-the-power-supply-o...