Giant Revel 29er 2015 - maximum width of tire I can fit?
The Giant bike specs page says nothing about tire clearance. You can measure it yourself:


Measure mounted tire width at widest point.
Measure distance from wide tire points from #1 to frame/fork.
Considering a (very arbitary) minimum 1cm clearance for 29er, choose a suitable tire.


For example, if current tire is 53mm wide and clearance at each point is 1.5cm, about 1cm wider tire might fit (about 2.3"). Keep in mind that tire width will vary from claimed depending on manufacturer and your rims inner width. You should also factor in riding conditions, especially amount of mud. And make sure that new tire will fit into rim brakes, if that's what the bike has.

You might find it easier to visit a local bike shop and find a suitable wider tire together. Just fit something bigger and see if clearance is good enough.