https://www.tennisicoach.com/en/content/tennis-anatomy-conditioning-for-the-arms-and-wrists.aspx
The size of the upper arms is not just the biceps. When "biceps" are perceived as huge, the triceps are usually responsible for a significant part of that size of the biceps. Also, not so sure that strength (or power) of the biceps a really of utmost importance for tennis players. Some strength/power is needed but I do not believe that we need to concern ourselves with developing that to a high degree for the biceps.
The primary functions of the biceps are flexing the arm (bending the elbow) and supination of the forearm (when the arm is flexed). The biceps also have an antagonist function for the triceps. For the most part, the triceps are employed in a more explosive manner for the serve and other strokes than the biceps. Triceps
power would appear to be more important than biceps power. (Note that power and strength are not the same thing).
It would appear that the development of the triceps is more important than the development of the biceps for tennis. However, this is not quite accurate either. When developing the triceps, it is also important to develop their antagonist muscles, the biceps, to avoid muscle imbalance. Developing the triceps w/o also developing the biceps would result in a potentially harmful muscle imbalance. Likewise, developing the biceps w/o also developing the triceps would result in imbalance.
Working with 5 lb weights for the biceps are probably fine. But it is important not to ignore triceps development. When performing biceps curl, be sure to include a rotation of the forearms (supination). With some triceps exercises, a forearm pronation can often be employed.
