"This is a database of blank minidiscs."
This is a Products table. You should have one record per product. And since you sell and price differently per size, you should one record PER SIZE (or three records in this case). In this way, the quantity applies to that specific size and counts are simple matter using a summary field on the quantity. If you leave it all in a single field, you will need 3 fields for the size, 3 fields for the quantity, 3 fields to summarize and so forth. That type of structure is a dead-end.