Congrats to you! That's one nice fish. That must have been one awesome fight to land it. Job well done!
My rule of thumb for trout is that a 24" fish is about 4 pounds. Add a pound for each additional inch of length. That gets you pretty close for average fish. Lean, skinny fish weigh less, pot-bellied ones weigh more. That one looks to be about average, so I'd say about 28 inches.