Height
Standard height for a coffee table is 16 to 18 inches. However, you should aim to make your coffee table fit your space and function. If the seat of your sofa is significantly higher or lower than that, adjust the height of your coffee table to within 1 or 2 inches of your sofa seat height. If you plan to eat off your coffee table regularly, you may prefer a slightly higher table.
Diameter
One of the most important factors in deciding the best diameter for your round coffee table is the scale of surrounding furniture. Your coffee table should be about two-thirds the width of your sofa. For an 8-foot sofa, a coffee table about 5-feet wide will look most proportionate. If you have an especially long, narrow space to fill, use two smaller round tables to fill the space.
Distance From Other Furnishings
Place your round coffee table about 18 inches from the sofa. However, with round coffee tables, you may move it as close as 12 inches from the sofa. Keep other chairs or seating areas that will depend on the coffee table for storage at about the same distance away.
Other Considerations
The table should always be large enough for everyone on the sofa to reach. If there's a seating area in your living room where a guest can't reach the coffee table, add another small occasional table nearby. Consider the function of the table and your situation. If you have small children, opt for a stone or distressed wood finish. If you primarily use the space for entertaining, glass or marble might be an attractive option.