It should be large enough to fit a sink and fixtures but not so deep you can't reach to the back. Typically a bar top that is a minimum of 18 inches deep works well, but you can extend this to 20 to 24 inches to allow more space for larger sinks and fixtures.
What are standard bar dimensions?
Home bar tops are usually between 12- to 16-inches wide. Commercial bar tops vary from 20- to 28-inches in width, including a drink rail (or trough) on the server side of the bar. Drink rails are typically 3/4-inch deep by 4-inches wide.