  Studying the long-term radio variability (timescales of months to years) of
blazars enables us to gain a better understanding of the physical structure of
these objects on sub-parsec scales, and the physics of super massive black
holes. In this study, we focus on the radio variability of 1157 blazars
observed at 15 GHz through the Owens Valley Radio Observatory (OVRO) Blazar
Monitoring Program. We investigate the dependence of the variability amplitudes
and timescales, characterized based on model fitting to the structure
functions, on the milliarcsecond core sizes measured by Very Long Baseline
Interferometry. We find that the most compact sources at milliarcsecond scales
exhibit larger variability amplitudes and shorter variability timescales than
more extended sources. Additionally, for sources with measured redshifts and
Doppler boosting factors, the correlation between linear core sizes against
variability amplitudes and intrinsic timescales are also significant. The
observed relationship between variability timescales and core sizes is
expected, based on light travel-time arguments. This variability vs core size
relation extends beyond the core sizes measured at 15 GHz; we see significant
correlation between the 15 GHz variability amplitudes (as well as timescales)
and core sizes measured at other frequencies, which can be attributed to a
frequency-source size relationship arising from the intrinsic jet structure. At
low frequencies of 1 GHz where the core sizes are dominated by interstellar
scattering, we find that the variability amplitudes have significant
correlation with the 1 GHz intrinsic core angular sizes, once the scatter
broadening effects are deconvoluted from the intrinsic core sizes.
