Does anyone know which one I would use with jelly rolls and layer akes, would it be the petite or the regular size one

It would be the petite, making the 5" HS. You would be cutting 5" squares, and the strips would be 1 3/4" & 2 1/4". Based on a layer cake being 5".

Personally, I'd use fabric not precut since it would need cutting anyway and to me that equals waste. Then, I'd do the 6" HS. cutting 6" [or 5 1/2"] squares, and strips of the same size as above.

AliKat - A layer cake is 10" square - it's the charm pack that is 5" square.