festae will need to get to at least 3-4" before you can be certain of sex.
Here is a link to my picture galleries. Check out all the shots of my festae pair. Unfortunately, there's only 1 or 2 shots of them when they're the size you're talking about, but dates are listed so you can see what they looked like at what age. Also you can see what to look for in the coming months to distinguish the males from females.
Just wanted to add that I bought them somewhere around March of '03.