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Background: There is an increased focus on randomized trials for proximal behavioral outcomes in early childhood research. However, planning sample sizes for such designs requires extant information on the size of effect, variance decomposition, and effectiveness of covariates. Objectives: The purpose of this article is to employ a recent large representative sample of early childhood longitudinal study kindergartners to estimate design parameters for use in planning cluster randomized trials. A secondary objective is to compare the results of math and reading with the previous kindergartner cohort of 1999. Research Design: For each measure, fall-spring gains in effect size units are calculated. In addition, multilevel models are fit to estimate variance components that are used to calculate intraclass correlations (ICCs) and R[superscript 2] statistics. The implications of the reported parameters are summarized in tables of required school sample sizes to detect small effects. Measures: The outcomes include information about student scores regarding learning behaviors, general behaviors, and academic abilities. Results: Aside from math and reading, there were small gains in these measures from fall to spring, leading to effect sizes between about 0.1 and 0.2. In addition, the nonacademic ICCs are smaller than the academic ICCs but are still nontrivial. Use of a pretest covariate is generally effective in reducing the required sample size in power analyses. The ICCs for math and reading are smaller for the current sample compared with the 1999 sample.