Combiner box selection starts with string count, but it does not end there. Fuse layout, SPD rating, isolator size, monitoring and enclosure space all change as the input count increases.
A 2-in box is not just a smaller 12-in box. The installation context and service expectation are different.
Small input boxes are often close to the array
2-in and 4-in boxes are common in smaller PV blocks or rooftop sections. They need clear labeling and enough room for field wiring, even when the circuit count is modest.
Large input boxes need more attention to heat and service
12-in and larger boxes should be reviewed with wiring space, gland layout, fuse access and internal temperature in mind. A compact box can look cheaper and cause installation pain.