will any shimano pedal fit any shimano crank? I bought a second hand boardman cb pro mountain bike. It came with no pedals (as expected) but the guy gave me some 1/2 inch screw pedals, flat basic ones. 

But they don't fit. The screw diameter is way too small for the thread on (I believe you call it a crank). 

The crank is shimano Fc-m542.

The bike is relatively modern. (within 5 years, at a guess). 

Is there a specific size pedal I need to look for so it fits tge thread / hole?
Normally, one piece cranksets use 1/2" x 20 tpi pedals. Three piece cranksets use 9/16" x 20 tpi pedals. Old french cranksets use something else.

You find one piece cranksets on old American bicycles or some very low quality bicycles (such as those you'd get from Walmart). Almost all other bicycles use three piece cranksets.  

So, you need a 9/16" pedal. Remember to grease the threads before installing the pedal and the left side pedal is counter-threaded (tightens counter clockwise) while the right side pedal is threaded normally (tightens clockwise).