In gvim, the size of the font is part of the 'guifont' setting, but
the value to set it to differs from one OS to the next. Happily, gvim
can determine which OS it is running in.
• For a short example of a code snippet which can be put in your
vimrc, see :help setting-guifont
• For a detailed explanation and a code snippet covering all OSes I
could find about, see
https://vim.fandom.com/wiki/Setting_the_font_in_the_GUI
• In both cases, the size of the font for Windows is the number after
:h in the value "for Windows". Increasing or decreasing that number
will make the font bigger or smaller respectively. You may, of course,
adjust to fit your taste whichever of the above-mentioned snippets you
decide to insert in your vimrc.
If, rather than gvim, you are using Vim in a console, then the console
(sometimes called the "DOS box" in Windows) determines the font to be
used and Vim has no power over it.
Best regards,
Tony.