work piece length width height in BOM

Best I can think of would be a binary parameter to say whether or not the part is cylindrical, and another parameter for the text to be displayed if that is the case. Then your repeat region displays the ModelCHECK parameter unless it's a cylindrical part. In that case it displays your alternative parameter.

Then you could have a mapkey that changes the binary parameter to TRUE and adds a relation to add the relevant dimensions into the string for the alternative parameter, allowing the user to pick what dimension corresponds to the diameter and what corresponds to the length.

Then, if you're making a cylindrical part, you need to run the mapkey and identify the relevant dimensions. If it's not a cylindrical part, you don't need to do anything.

Alternatively, use a different start part for cylindrical parts that has a pre-modelled cylinder and contains the aforementioned parameters from the start. So if you're making a cylindrical part, you use that start part and just adjust the first cylindrical feature's size.