Hi Steve! Welcome to the forum. When you get a minute, stop over by the intro thread and say howdy.
In re: your revolver- the value of any gun is based on EXACT make, model, and condition. There are two very close (but different) versions of that Colt revolver- the Model 1889 and the 1892- and there is a first, second, thrid issue, etc. Difference between a 70% and a 90% is substantial.
Just as a really wild, stab in the dark, mildly educated guess, your Colt may be worth $1000 to $2000. You REALLY need a hands on assessment from an old Colt expert.
Re: barrel length- open cylinder. Place a flat obect like a ruler across the opening at the rear of the barrel (the forcing cone) Slide a rod or dowel near to bore size into bore until it stops at the ruler. Mark it at the muzzle, withdraw, measure.