i don't think they feel cold but i have never experienced that with regular aluminums either so i may just be missing it.
personally, i would generally start with 16" because i find them hardest to find in most stores. (and you could possibly take those hats off DPNs
But of course that is just my opinion. the 24" will definitely work on both the bag and the scarf--will NOT work on the hats unless it is for someone with a very big head or very big hair. (or if it is knit flat!)
longer needles are good for 2 circ knitting and magic loop knitting.
so yeah....i am no help but my opinion is 16"!