Some sources report that there should be a 1:1 strength ratio between the biceps and triceps. That is, the biceps and triceps should be of equal strength, so if you use 20-lb dumbbells for biceps curls you should also be able to use 20-lb dumbbells for triceps extensions. This concept looks nice on paper, but I find that it rarely applies in real-life situations. Structurally, the triceps is a larger muscle than the biceps but we tend not to use it as much as the biceps in activities of daily living. For example, pulling tasks such as carrying objects, lifting food and drink to our mouths, and pulling open doors all require the biceps to contract. While we do perform opposing pushing movements that work the triceps, such as lifting objects overhead and pushing open doors, we tend to do those less frequently. The point being that most people, especially novice exercisers, can lift more weight with their biceps than their triceps.
Rather than focus on a strength ratio, I suggest determining the amount of weight that it takes to challenge each muscle group upon the completion of between 8 to 12 repetitions. This way, your muscles will be worked appropriately based on your own individual level of strength instead of an objective weightload. Then consider working your way toward a more even 1:1 strength ratio between the two muscles, keeping in mind that with the triceps being the larger muscle, they might surpass the biceps’ strength over time.