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.
Yeah so if you are keeping Windows for, like, gaming and stuff I bet you'll be fine. I think outside of maintaining your Linux side updates, the only real annoyance you'll have is switching OSes when you want to.
I dual-booted back in 2015 I think but finally made the full plunge to Linux after some hardware upgrades nuked my Windows side. But again, if this is a new build, I don't think that will be an issue. And last I played with it, Win10 is better at hardware upgrades now.
Long and short, you'll probably be fine.