There are these types of fonts in latex:

\tiny
\scriptsize
\footnotesize
\small
\normalsize
\large
\Large
\LARGE
\huge
\Huge

Generally speaking, the default font size is \normalsize. We can redefine the default font size at the beginning:

\documnetclass[12pt]{article}

Just change the value of 12pt. Latex provides three sizes: 10 / 11 / 12pt

Or directly define the text size:

\fontsize{5.0pt}{\baselineskip}\selectfont  text

Change 0.5 to the number you want.