Brian,
I prefer to ultrasound for sex determination prior to winter cooling at their second year or the following year. This is usually their first possible reproductive season and the follicles in females should be fairly easily visible. There is no target weight or size. It depends more on how quickly they are grown. The first reproductive season could be at two years old on a quickly grown animal or four years for a very slowly grown animal.
My book outlines the technique I use, including photos of testes and ovaries as seen on ultrasound. Many people have told me they have taken the book to their vet as a guide and have been successful. Certainly a vet that has done the procedure a few times should have no problem, but even a vet that has never done it can be successful if the right time of year is selected.
Mark
www.DrSeward.com
