How to change document font size in preamble from 11pt to say 12pt?

Would something like this do?



my report here

In the KOMA-script classes this can be easily done thanks to the \KOMAoption command, which allow a class option to be determined outside \documentclass, whether inside or outside the preamble. (If issued outside the preamble though, the fontsize option declaration will take effect from that point onwards.)

Some 12~pt-sized text (default is 11~pt).

With a standard class you can load package scrextend. Then the fontsize can be changed while loading this package or later on by \KOMAoption or \KOMAoptions.

my report here
\par \bigskip
my report here