Assuming that you’re a male, you would most likely be able to play a low G, low F (F is so much prettier than G, for some reason, in my opinion, but I just can’t explain it), or even an E/Eb and D. It just depends on the make of whistle (low Susatos are evil) and the size of your hands.
I have normal sized hands for a male and can play a normal sized Bb very comfortably without piper’s grip. I haven’t tried lower keys yet but I would imagine that my fingers can stretch a few keys lower.
I have average size hands for a male. I play a low d, and i havent found any problems with fingering. although I do use a slightly modified pipers grip, I still use the tips or pads i guess you would call them of my fingers, not the 2nd knuckle like all the pictures show. but I have only been playing for a little while. I have no trouble reaching everything. I started on a high d and went straight to low d, now i find that i have trouble when i pick up the high d. something about too close together
Note that the lowest whistles like D vary a lot in their finger stretch. My wife can’t play my Kerry Pro low D, but she can play my Sweet pretty easily, even with the lack of about 1/2" of her left ring finger. If you want to go all the way low, this is an option.
I don’t think it’s the size of the fingers so much as it is the comfortable stretch. I don’t have freakishly long fingers, but I play low D’s fingertip style without any issues (though I use my pinkie on the lowest hole).
I currently own two Low D whistles and they are very individual.
Chieftain Low D. I’ve had this one a few years. Pipers Grip is almost a must on this one. I have some fat fingers, but I can’t really cover the holes reliably unless I use a Pipers with it.
Copeland Low D. I can easily play this one with my finger tips. The stretch is there, but nothing I can’t overcome. If you have smaller hands, you may have to use a Pipers on this one also though.
I have a Low A and a Low G Water Weasel Set, and neither is a stretch to play as normal.
I think this can be very personal. I have trouble playing a Bb but no trouble with a G. I can play low D easier than the Bb as well. Like most things the more I play the Low D the easier it becomes. For reference I have a Jubilee Practice Low D and a Howard low D. These are two very different whistles in hole size and reach. My Bbs are Generatio and my G is a Dixon tunable.
I have medium sized male hands (broad palms, but fairly short skinny fingers).
I can play high-whistle fingertip style on my Chieftain and Serpent A whistles just fine. Ditto for my only F whistle, an older Serpent all-brass model. (I was also able to fingertip a couple of Overton F whistles friends let me try).
For me, the F whistle is the crossover point - depending on my mood, I can play one either high-whistle style or with piper’s grip. But I usually don’t bother with piper’s grip on the Serpent F (this might not hold true for other makes and models).
I can play my Dixon Low D fingertip-style, also, but I usually prefer piper’s grip. I find piper’s grip a necessity on my Howard Low D, though.
Hand position has a lot to do with it, though - if you angle your whistle out to right (about halfway between centered and “flute” position) it makes the stretch a lot easier.
All that being said, though, piper’s grip is a lot easier on your hands, and is (IMHO) well worth taking the time to learn.
I have a relatively small hand, my finger to finger maximum reach with a normal grip is 3 inches and I find that the Bb is as much as I want to stretch without goint to the piper’s grip. Just go ahead, and lay your fingers on a ruler as if you were holding a whistle, and see what you get, then compare with whistle sizes. My Generation Bb, F#-D holes are 2 inches appart. That means that you should be able to play relatively comfortably with a hole spacing equal to 2/3 your maximum reach in your normal finger position (you may be able to reach 3 inches, but can’t lift a finger!)