If this is the motherboard of your HP then it's an ATX form factor one.
Here's a picture: https://www.google.ro/search?q=Motherboard+506285-001&client=ms-android-huawei-rev1&hl=en&prmd=ivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiy-LrgovHeAhUOC-wKHRjgBp4Q_AUIECgB&cshid=1543208373362&biw=360&bih=600#imgrc=oWBtB_PGPWUDxM