The junction box's standard size starts from 4×4 inches. Imagine a cross-junction with horizontal and vertical straight pulls of wires from ½-inch conduits to ½-inch conduits.
According to the 2020 National Electrical Code, the length of a junction box must be at least eight times the largest conduit for straight pulls. Because of that, we get ½ inches × 8 =4 inches for our box's minimum horizontal and vertical dimensions. Read more