Just my opinion, but i found the smaller they are, the easier they are to probe.... they have less muscle tone to resist and therefore make it far easier to do as you like without resistance...
Angel
Are you speaking from experience angel?How many hatchies and older animals have you probed?It's not hard to stuff up and turn a male into a female as indicus stated.....I would suggest unless you're a big breeder or are in a real hurry to know the sex of a snake and have the experience to do so to wait till they're a bit bigger than a hatchie.
There is also a way i've been shown without actually probing to sex an animal...hard to explain and i think popping them is cruel!!!