Have all the products at hand.
As for size, they have disposable gauges and will direct you to a private room where you can make the measurement.
The Coloplasts are excellent quality, but come with glue that goes on much easier than they come off
They are made for paraplegics with no sensation.
Unroll the things and remove the glue with laquer thinner. Dust with talcum powder and roll up until required.
Use a bit of electric tape to secure behind the corona - not too tight. Run plastic tubing to a freezer bag.