The following minimal working example explains how to do so:

% Frame number (font size)

% clear the predefined navigation symbols

% set a bigger font for the page number
\setbeamerfont{page number in head/foot}{size=\large}

% print only the frame number in the footline
\setbeamertemplate{footline}[frame number]


    This frame is a test to see how the footline looks.


Compiling this code yields the following output:

font size of Frame number in beamer
How the customized footline, with the big frame number, looks

Observe that, as we have done in many other examples, we have changed the footline element of the beamer template. In this case, we have also changed the theme font for the page number in head/foot, only modifying its size (but we could have gone further and changed its family, type, and so on).