Welcome to the forum
@saildad4.
The question is a difficult query. Not all boats are the same. Fortunately the cost of flax stuffing material is cheap. I’d buy a couple of sizes.
3/16” to 5/16”. I think your boat will use something on the smaller size. My shaft is 1 3/8 diameter and I use the 1/4” size.
With the boat out of the water and your stuffing box available to work on, you can open it up, pick the old stuffing out, take it to the chandler and find a match.
Here is a good link that covers the adventure you are beginning.
The Stuffing Box (click any image to see a larger version) While digging around in some old boat parts boxes I found a stuffing box from an older sailboat. I though it would be a good time to rig it up
marinehowto.com