What size fork can accommodate this bike that is larger than the normal one? My bike is a 2019 Boardman Mtr 8.6 and I’m wondering if I am able to fit a 140mm travel fork to my bike? 
Here’s a link to the bike and it’s specs:
https://www.cyclerepublic.com/boardman-mtr-8-6-mens-full-suspension-mountain-bike-2019.html
You can swap out a suspension fork for a different one with more of less travel as long as the new fork has approximately the same axle to crown distance - the distance between the center of the axle and where the fork mates with the bottom of the head tube.

IF this distance is too long the frame is tipped back and the steer angle slackened, if the fork is too short the frame tips forward and the steering angle is steepened. Both can have negative effects on handling.

Longer travel suspension forks tend to have longer axle-crown distances as obviously they need more room for the front wheel to move.

Note that many forks are designed to compress with the rider on the bike. The compressed axle-crown distance is whats important.