Slug you bore and chambers.
Use a fishing weight - which is usually pure lead.
Pick one that's close to the same diameter but a little larger.
Drive it through the bore using a wood block as a mallet and a dowel rod.
Measure the diameter of the fishing weight between the high spots the rifling leaves on it. That's the diameter of the grooves in the barrel.
Next, take 6 more fishing weights and drive one through each chamber.
Make sure you support the cylinder so you don't spring the crane.
You can just open it and place it on the edge of a table.
Measure the diameter of the weights for each chamber.
For lead slugs, you generally want to stay @ .001 over bore size.
If you bore slugs @ .357, then use .358 bullets.
Most .357's out there are going to slug out @ .357", so you're pretty safe to go w/.358 and not go to the trouble of slugging the bore.
It's still a good thing to know though in case you have accuracy or excessive leading @ some point.