1 cup of chopped onion is the middle one

Most recipes will say 1 onion and give you no amount. Use the medium one in the middle (about 2 1/4″ diameter), unless you like more or less onion, then of course use a smaller or larger one.

 How to Slice an onion or shallot:

Slicing Onions

1.  Slice the onion in half and using the remains of the stem, pull the skin off from the top
2.  Trim the stem and ends and notice the grain of the onion
3.  Slice the onion WITH the grain until you start to get nervous the knife is too close to  your fingers
4. Flip the remaining piece down.
5. Wow, much easier to slice that.
6. Your perfectly sliced onion.

When you slice with the grain less volatile chemicals are released that combine and make you cry.

1. If you wish to chop, dice or mince, again start cut side down. Trim the remaining stem end off, leaving the root intact. There will be less crying that way. A sharp knife also helps.
2. Make cuts like the spokes of a wheel around the onion cutting almost to the root end. You want the onion still intact so it is easier to cut.
3. Turn the onion 90° so you are cutting opposite (perpendicular) to the first slices.  The width of the cuts determine the side of the pieces.