When you change the font size, it's changing a per-window property, called p_font_size.
I don't see a built in way to show that variable. This shows it:
#include "slick.sh"
_command void show_font_size() name_info(','VSARG2_READ_ONLY|VSARG2_REQUIRES_EDITORCTL)
{
_message_box('Window font size is 'p_font_size);
}
It gets persisted in vrestore.slk, since it's associated with a particular window&buffer.
It can persist across SlickEdit startups if the buffers are restored on startup. If you've enabled settings so no files are restored on startup, the changes aren't restored.
It is not used for new windows.