I plan to do a pc upgrade very soon. Alongside that I plan to start with Mint or Fedora. Is there any real or big downsides to dual booting ? Aside from the harddrive space lost ?
If I like or really like my time with Mint I would probably switch permanently but I felt like I wanted to test it for a couple of months before making a complete switch.
Im a big 
so that impacts my consideration. Dont really play much multiplayer shooters so I dont have a problem with kernel anticheat games not working.
Not really other than hard drive space lost. fwiw when I first installed Linux I did a dual boot setup but ended up never booting into Windows except to play games, but I just played non-Linux games very infrequently because it wasn't worth it to me to reboot my whole computer just to play a video game. And nowadays the vast majority of games run just fine on Linux, either due to native Linux support (you'd be surprised at how many popular games have native Linux versions) or through Proton (a compatibility layer made by Valve that allows you to run Windows executables on Linux).
I guess the main downside is, if that's you, and you later decide to make the whole drive Linux, I have in the past had problems with data corruption when I try to expand a partition after deleting another. It is a potential source of problems, as something could always go wrong with repartitioning, but I imagine the actual risk is still fairly low.
But if you're sticking with a dual boot setup then it shouldn't have any real "downsides". Windows will work as normal and Linux will work as normal.