What is the min. size of the Hard Drive to install Ubuntu? I am a new user

Based on the documentation from Microsoft and the Microsoft Store, you will need:

• Windows 10 version 16237.0 or higher
• ARM64 or x64-based CPU
• Keyboard
• Mouse (probably unnecessary)

Speaking from experience, you will want to run WSL on a machine that has at least 8GB of RAM and 30GB of free space. The hardware specifications you outlined for your HP notebook are more than enough.

Do note that the initial installation is just a few gigabytes in size (450MB download / ~3GB installed), but a WSL container will not (yet) auto-shrink as files are deleted. So make sure that there's enough free storage space on your machine to handle this limitation.