Conventionally, the maximum aggregate size is determined by the sieve size that retains at least 15 percent of the particles in a sample set.

As the maximum aggregate size increases, the surface area per unit mass that must be wetted or coated by the cement paste decreases, improving the workability of concrete.

However, beyond a specific size, larger aggregates offer less area for bonding with cement mortar.

Larger aggregates also tend to produce more microcracks at the coarse aggregate-cement interface and introduce discontinuities in the concrete mass, leading to a heterogeneous concrete with reduced strength.

The maximum size of the aggregate depends on the dimensions of the concrete member and the clear spacing between the reinforcing bars in the concrete member.

Additionally, the cost associated with stockpiling aggregates of different sizes required for a stable mix and preventing segregation while handling and placing these aggregates is vital in deciding the maximum size of aggregate. 

Generally, for structural concrete, the maximum size of aggregate is restricted to one inch or one and a half inches.