Designing metallodielectric nanocomposite materials for photonic applications implies the ability to reliably model and predict the effect of metallic inclusions on the optical properties of a microstructured composite material. We present a detailed analysis of the optical properties of block copolymer based, multilayered metallodielectric Bragg reflectors and discuss the implications of nanosize-specific effects on the design of composite materials for photonic applications. A series of lamellar poly(styrene-b-ethylene/propylene) copolymer thin films with varying filling fraction of gold nanocrystals preferentially but randomly distributed within the poly(styrene) domains have been studied by reflectometry. The reflective properties are compared to model calculations based on effective medium concepts taking into account confinement effects on the dielectric function of the nanocrystals. The particle size causes strong alterations in the dielectric function of the metal inclusions that have relevance for the attainable dielectric contrast within periodic nanocomposites.