What size, dunno - the wider the hollow space and the longer, the deeper the sound will be (think the size of a piccolo vs a tuba). Definitely the dried stalks, you wouldn't want your good whittling to go to waste if a worked-on green stalk warped as it dried, or split. But you might try putting plugs (wine corks?) in green ones you want to work on - as the stalk dried around the plug it should tighten it nicely. You want it to be air-tight except for two holes: where you blow into it, either at one end or on the side near a closed end, and a bigger hole for the air to escape. If you're really, really good, you can vary your notes just by the way you blow. Otherwise, make high or low notes by making several holes 1/4 to 1/2 inch in diameter along the length, so you can stop them up or open them by lifting your fingertips up and down. I can remember my dad making whistles for us from stems he cut from trees or bushes he knew had pithy centers. They'd be 5-6 inches long, maybe half an inch thick. He skinned the bark off, pushed out the pith with a skewer and made a few holes with his brace and bit. Not sure we sounded very musical, but it was fun. Your flutes would be similar, just bigger and no doubt more pleasant sounding. I've not studied sunflower stalks - do they get woody enough to carve? Strong enough to lean on? You could carve your flutes, or make walking-canes.