You want to play around with the \setbeamerfont command.
- You can direct commands for the whole document at various "parts" of the Beamer doc
--- Scan the manual for all the part names you can fiddle with but for sure these are frametitle (title on each slide), title (title on the first slide), author (author's name on first slide), footer/header (the stuff at the bottom/top of each slide)... and probably subtitle? I forget all of them and haven't found a great concise list yet...
/--------
| \setbeamerfont{parent A}{size=\large}
| \setbeamerfont{parent B}{series=\bfseries}
| \setbeamerfont{child}{parent={parent A, parent B},size=\small}
| \normalfont
| This text is in a normal font.
| \usebeamerfont{parent A}
| This text is large.
| \usebeamerfont{parent B}
| This text is large and bold.
| \usebeamerfont{parent B}
| This text is still large and bold.
| \usebeamerfont*{parent B}
| This text is only bold, but not large.
| \usebeamerfont{child}
| This text is small and bold.