Along with your builder, you can determine a gunite pool size by considering available backyard space, desired pool shape, pool style, features, accessories, and plans for any outdoor amenities.
How your family plans to use a pool is also important: For example, if you want a pool for swimming laps or a pool with a diving board, a minimum length is typically required. Likewise, including a beach entry and a big custom spa for a family of six will dictate a specific length/depth requirements.
On the other hand, if you just want to jump in the water to cool off, a compact splash pool will suffice.
You can learn more here about planning pool size:
Swimming Pool Design Dimensions: 3 Key Initial Questions to Ask
Concrete Pools Ensure Best Versatility for Size Shape & Configuration