How to find the best bullet dia for revolver?

I know Gunsmith has a lot of information on this subject, send him a PM. I don't think he checks here often but i know he would be happy to help

Irelander - You are not over thinking the situation.
You are correct in thinking you need to look at both the barrel bore diameter and the diameter of the exit hole in the cylinder.  
The basic approach is to determine the barrel bore diameter via slugging.  You also need to determine whether there is a barrel constriction (i.e. a tightening or choking down in diameter) where the barrel threads into the frame - sometimes the barrel will be smaller there as it gets mis-shapened when the barrel is over tightened into the frame.
Now knowing what your barrel diameter is, you can check that your cylinder exit holes are not smaller than the barrel.  Try to imagine what happens to accuracy if they are smaller - the cylinder exit holes swage down the diameter of the fired bullet, then the smaller-than-bore-diameter bullets "rattles" down the larger bore of the barrel.  This causes accuracy and leading problems (leading because you get more gas "blow by" around the smaller bullet).
Most articles I have seen seem to say that the cylinder exit holes should be about 0.0005 larger than the barrel bore diameter.  This will insure that the bullet arrives in the barrel a little larger than the barrel bore and thus seals tightly.  As an example, a 45 Long Colt revolver with a barrel diameter of 0.452 would enjoy having cylinder exit holes of 0.4525.
Having the cylinder exit holes reamed to the proper diameter has really helped some of my Rugers in the leading and accuracy department. 
All the above is a very basic look at what is going on.  I am sure there are others here who can give you much more detailed info on the subject.
If you ever wander over to the two Ruger single action forums (www.rugerforum.COM and www.rugerforum.net), you will find much discussion of this topic over the years.

There's also some measurement data on chamber mouth dimensions on similar revolvers on page 3, post titled ".44 Chamber Mouth Dimensions".