Electromagnetic and thermal simulations of a range of models (body mass 14-70 kg) were performed for a 7T (297MHz) birdcage head transmit coil. Head averaged SAR (hdSAR) was found to be the limiting factor for all cases, and a (weak) negative linear correlation with body mass was observed (i.e. hdSAR increases with decreasing body mass). Thermal simulations with fixed blood temperature (assumes good thermoregulation) predict temperature increases remain within guidelines if SAR limits are respected. If blood temperature is allowed to vary then elevated core temperatures are predicted for subjects with smaller body mass even when SAR limits are respected.