Randomized experiments are often considered the strongest designs to study the impact of educational interventions. Perhaps the most prevalent class of designs used in large scale education experiments is the cluster randomized design in which entire schools are assigned to treatments. In cluster randomized trials (CRTs) that assign schools to treatments within a set of school districts, the statistical power of the test for treatment effects depends on the within district school-level intraclass correlation (ICC). Hedges and Hedberg recently computed within-district ICC values in eleven states using three-level models (students in schools in districts) that pooled results across all the districts within each state. While values from these analyses are useful when working with a representative sample of districts, they may be misleading for other samples of districts because the magnitude of ICCs appears to be related to district size. To plan studies with small or nonrepresentative samples of districts, better information are needed about the relation of within-district school-level ICCs to district size. Our objective is to explore the relation between district size and within-district ICCs to provide reference values for math and reading achievement for grades 3-8 by district size, poverty level, and urbanicity level. These values are not derived from pooling across all districts within a state as in previous work, but are based on the direct calculation of within-district school-level ICCs for each school district. We use mixed models to estimate over 7,000 district-specific ICCs for math and reading achievement in eleven states and for grades 3-8. We then perform a random effects meta-analysis on the estimated within-district ICCs. Our analysis is performed by grade and subject for different strata designated by district size (number of schools), urbanicity, and poverty rates. [This paper was published in: "Evaluation Review" v38 n6 p546-582 Dec 2014.]