While it's hard to know exactly because it varies by size, a medium onion minces to about a cup. Depending on the size and how finely the onion is minced, it may yield up to 2 cups.
Onion powder or dried onion may work as a good substitute for diced or minced onion. Using these replacements often found in the dry spice aisle can help cooks avoid the tears and mess associated with fresh onions. If the taste and flavor of a fresh onion are preferred, there are many tools that will help minimize the trouble, such as a hand-held vegetable chopper or a food processor.