Quite large, actually. A motor has to ingest a significant (huge) amount of fluid to hydrolock. Hydrolock occurs because you can't compress a fluid. Once you've filled a cylinder beyond its minimum volume with a fluid the motor effectively locks. So you'd basically need to consume 70+ccs of a fluid (size of combustion chamber) into ONE cylinder to cause a hydrolock. 70cc is ~ 2 fl oz. So basically you'd have to get ~10% of the bottle in almost instantenously to cause hydrolock. Very, very, very unlikely to happen. The odds are much better that the motor will stall before hydrlock actually occurs if you're following the directions.
Wow thank you, that’s was an incredible useful illustration, and excellent point. I think I may just go out and try this....:beer