Slightly generalized:
final TabWidget tw = (TabWidget)mTabHost.findViewById(android.R.id.tabs);
for (int i = 0; i < tw.getChildCount(); ++i)
{
final View tabView = tw.getChildTabViewAt(i);
final TextView tv = (TextView)tabView.findViewById(android.R.id.title);
tv.setTextSize(20);
}