A general rule of thumb is to be able to squeeze your index finger between you and the belt. That way, you’re giving yourself enough cushion to breathe out and brace your core. If it’s too tight, then you’ll have a harder time activating your core, and you’re liable to disrupt your breathing pattern, causing you to tire out quicker and even miss your lift. A 7mm belt is a good choice if you want greater mobility and range of motion when lifting compared to the thicker belt choices. However, it may not provide as much support which could increase the risk of injury when performing repeated heavy lifts. It may also be illegal to use in some powerlifting competitions depending on the federation.