Hybrid Bike tire selection what size tire can install on my bike? I have hybrid bike (gary fisher Tiburon 2006). 
It has 700x35C tires on it or if you want to call it 37x622. 
I want to install the widest tire possible to ride on semi rough trail.
Today I received my new tires 700x42c but I wasn't impressed with the unnoticeable added width of 7mm as if there is no difference in size. 

I have lots of clearance on all the relevant tires and frame to accommodate wider tires. I have about 3/4" on both sides of the front forks and rear frame. Also enough space between the brakes holder and rim. I checked with a bike shop and they advised me that I have enough room for much wider tires.

Please advise the widest tire size I can install on my wheels.
If you have a 42mm tyre, and still have 3/4" of clearance at all possible points, then your maximum size would be 42mm +19mm = 61mm.

Do check the clearance at the chainstays, the seat stays, and the brake bridge.  You imply you have rim brakes, they're often the limiting factor.