How do I determine what size file to use?

Hi MarleyJeanne,
You first have to know the pitch of the chain. You can match that number (in inches) to the diameter of the required round file size.

-- .404 chain use 5.5mm (7/32”) round file
-- 3/8 chain use 5.5mm (7/32”) round file
-- .325 chain use 4.8mm (3/16”) round file
-- 3/8 lo pro chain use 4mm (5/32”) round file

The bar says it's a 325 itch but I don't see any #'s on the chain itself that tells me anything--on the chain (Oregon) the top tooth has #25 and bottom tooth has #22..

Take the saw to HD, Lowe's or Ace. They can easily match the file to the size needed. I use a 7/32 stone in a Dremel. You also need to do a depth check on the rakers. If the chain gets cut too much, the untrimmed rakers will bottom out and the chain will not cut. They have a tool for that and you use a fine flat file, not a chain file.

The bar tells you what the pitch of the chain is. You can't put the wrong chain on the bar, so according to what you said, use 4.8mm (3/16”) round file.

When I wanted to replace the chain on my saw, I simply googled the model of the saw and got the manufacturer's page listing the parts.