1. LaTeX font size
command 8pt 9pt 10pt 11pt 12pt 14pt 17pt 20pt
tiny 5pt 5pt 5pt 6pt 6pt 6pt 8pt 10pt
scriptsize 5pt 6pt 7pt 8pt 8pt 8pt 10pt 12pt
footnotesize 6pt 7pt 8pt 9pt 10pt 10pt 12pt 14pt
small 7pt 8pt 9pt 10pt 11pt 12pt 14pt 17pt
normalsize 8pt 9pt 10pt 11pt 12pt 14pt 17pt 20pt
large 10pt 10pt 12pt 12pt 14pt 17pt 20pt 25pt
Large 11pt 11pt 14pt 14pt 17pt 20pt 25pt 29.86pt
LARGE 12pt 12pt 17pt 17pt 20pt 25pt 29.86pt 35.83pt
huge 14pt 14pt 20pt 20pt 25pt 29.86pt 35.83pt 42.99pt
Huge 17pt 17pt 25pt 25pt 25pt 35.83pt 42.99pt 51.59pt
2. Change font size
(1) list items and subitems
\setbeamertemplate{itemize/enumerate body begin}{\large}
\setbeamertemplate{itemize/enumerate subbody begin}{\tiny}
(2) footnote
\setbeamerfont{footnote}{size=\tiny}
References:
[1] What are all the possible first arguments to \setbeamerfont?
[2] Highlight only current subsection, hide subsections of other sections
[3] beamer - Changing font size of list items and subitems - TeX - LaTeX Stack Exchange