One of us recently visited a patient with a two-year-old colostomy. The patient was suffering from severe skin irritation caused by using appliances with pre-cut stoma openings of the same size as originally measured in the hospital.
Immediately after surgery, the stoma is quite swollen; it then shrinks for about the next six months--sometimes for a year or longer. During this initial period, it is best to use a cut-to-fit appliance and measure your stoma every time you change the barrier. Once your stoma size has stabilized, you may switch to a pre-cut appliance if you wish; however, you should continue to measure your stoma occasionally, to see if you should switch to a different size appliance.
If you fail to adjust your appliance size as your stoma shrinks, you'll eventually be using an appliance with an opening much bigger than your stoma. This leaves a large area of unprotected skin around your stoma, making you a prime candidate for skin irritation.
If you've had an ostomy for many years, perhaps you've forgotten that initial period while your stoma was shrinking. However, if you find yourself visiting a new ostomate, this is a topic you may wish to discuss if you have the chance.
How big is the optimal appliance opening? For most types of barriers/faceplates, the opening should provide clearance of a millimeter or two all around the stoma. On the one hand, you should minimize the area of unprotected skin around the stoma; on the other hand, some clearance is usually necessary because many barriers contain hard materials (including plastic films) that can damage the stoma if they come in direct contact. Please note, however, that some of the newer barrier materials, such as Hollister's Flextend® and ConvaTec Durahesive®, are more protective and contain no hard materials, so they don't require any clearance.