You can probe any size baby, as others have pointed out, the important thing is just what size probe you use. It may be easier for you as a first timer to try probing a juvenile (1,000 gram or so) rather than a baby or full sized adult. A juvenile is big enough for it to be easy to sex, and small enough to be easy to control.

Babies IMO are easier and faster to just pop rather than probe, and adults can be a bit of a handful for a beginner trying to learn. Try a juvenile, and go from there!

-Jen