Some manufacturers show a color code with a barrier size. For example, a Coloplast barrier with a dot tells its millimeters & color code. Their #10551 cut-to-fit shown here is 35mm barrier, and GREEN. When that part became a 25mm Pre-Cut, the barrier went to 40mm and color code remained Green. This can change the overall fit while undetected by color code.
When a brand & barrier with your Cut-to-Fit is established, confirm those dimensions are retained when changing either; different versions require testing with an ileostomy.
Some photos aren't exactly the part they claim, and are not to scale.
Pre-cut sizes are specified.
Barrier millimeters will differ by manufacturer, maybe to flange ID, flange OD, to pouch contact area, etc.
I'm using smallest n available
with a 1-piece, flat pre-cut.
The more I know about a few dimensions, the better
my next shopping spree may be.