To determine the proper dimensions, measure the space that's around your dining area. Your diners must be able to sit down and get up easily from their seats. Leave approximately 42 to 48 inches in between the walls and the dining table.
You shouldn't just measure the clearance between your walls and the table, but also the clearance between the furniture and your future table. If there are furniture pieces in your dining area, start the measurement of 42 to 48 inches from the furniture's edge instead of the walls. Do this and your task will be easier and quicker.
If you want the unconventional way of measuring, you'll be able to test out your table's size within the room by grabbing one of your bed sheets. Place it where you'll place the glass top wrought iron dining table. Then, fold it into your table's shape. By doing so, you'll be able to visualize the space the table will take. You can then measure the table-to-wall clearance as well as the table-to-furniture clearance.