Simply unscrew the collar lock nut and measure the inside diameter of the stuffing box.... If the boat is still in the water - shouldnt be leaking 'that' badly when you remove the lock nut; but, expect a 'flow', .... you can at the worst hold the packing inside the stuffing box with one hand as you measure with the other. For the first time better to do this job with the boat hauled out of the water.

Inside diameter of stuffing box MINUS shaft diameter .... all divided by 2 = packing size. 'closest' 1/16 inch is 'good enough'.

GFO all PTFE braided packing is absolutely the 'best' with which to replace.