The jacket should drape cleanly over your chest and down to your midsection without the interruption of wrinkles or folds in the fabric. There should be enough room in the chest to allow the lapels to lay flat. If the lapels bend outward and create a space between the jacket and the shirt, the chest width is too tight.
You can pair a blazer with jeans, a V-neck shirt, and loafers for a casual date night look, wear a blazer to a dinner party with chinos and a white shirt, or sport a blazer at the office with dress shoes and a dress shirt if your office dress code allows.