  We calculate the kinetic-decoupling temperature for weakly interacting
massive particles (WIMPs) in supersymmetric (SUSY) and
universal-extra-dimension (UED) models that can account for the
cold-dark-matter abundance determined from cosmic microwave background
measurements. Depending on the parameters of the particle-physics model, a wide
variety of decoupling temperatures is possible, ranging from several MeV to a
few GeV. These decoupling temperatures imply a range of masses for the smallest
protohalos much larger than previously thought -- ranging from 10^{-6} earth
masses to 10^{2} earth masses. We expect the range of protohalos masses derived
here to be characteristic of most particle-physics models that can thermally
accommodate the required relic abundance of WIMP dark matter, even beyond SUSY
and UED.
