You can calculate the size by taking two measurements: the diameter of the prop shaft and the diameter of the threads behind the packing nut. Subtracting those two and dividing by two will give you the packing size. You probably should use calipers for these measurements. It's a good idea to have a set of 0-6" digital, dial or vernier calipers aboard for various precision measuring tasks.3/16" or 1/4". How do you know?
You're welcome, Jim. Effort! Our boat has the standard bronze stuffing box with a 1-1/4" prop shaft. No way can I get 1/4" into that box unless I hammered it in. I've been using 3/16" for years (seven to ten years) with good results and even tempted to go with 1/8" because of how difficult it was to install 3/16". Your picture appears to be of the same product that I use. Low friction, good seal, lasts for years.How do you know?