This isn't very helpful, but it really is hard to say - the limit on the website is the limit, and anything more than that they could stop you.. the underseat area on ATR planes is smaller enough that if you have one of those bags that's designed to fit exactly into underseat storage it'll probably be physically too big. That said, the dimensions on their website are conservative. Realistically, a large laptop bag or backpack of the kind you'd happily sling over your shoulder and carry around all day would probably be fine. The sort of bag that you'd drop at the hotel before going out would probably be too big. In general, the types of bags you'd typically see on planes in the US (both overhead and underseat) would be way too large.

I carry a large laptop bag as my personal item, never blinked at it (although they have complained about my overhead bag which is 2cm too tall!)