I'm trying to understand what the "size" column means in options chains.

CME says that the size column "displays the volume/number of contracts of the trade or the size of the bid/ask."

I'm not sure I understand this definition. Does it mean that this is the number of contracts the market maker is willing to write at the current ask price?

The size columns in your image are the quantities associated with the "Bid" price shown, and the quantities associated with the "Ask" price shown.

For the 104 strike, there are 115 contracts available at a bid price of 0.15, and 719 contracts available at an ask price of 0.20.

Dave