The motherboard says its
- Customized: 29.09 x 20.3 cm (11.45 x 7.99 in)
So what should I research about it? How much space it takes in the case? I'm pretty new to tech stuff and I still don't get what I should research
I think the dimensions of a standard ATX motherboard are:
12 inches x 9.6 inches (305 mm x 244 mm)
That's a lot different than your HP board, part of their "customization".
The use of non-standard proprietary parts is a major reason to avoid pre-built PCs. You never know what level of customization you might run into and how or if you can work around it.
The HP case may accept an industry standard ATX power supply.....or not. Even if it did, the cables on the standard PSU may not match possibly proprietary connection points on the proprietary motherboard. The HP PSU may itself have weird dimensions apart from its quality or power rating.
You might be able to research what you need to find out in an hour on the Internet. Or you might spend 50 hours without learning much. At some point, maybe you change your plans and move to industry standard parts or live with what you have. Not an ideal situation.