For all new workbooks.
Tools>Options>General>Standard Font>Size.
For existing workbooks, right-click on any sheet tab and "select all sheets"
to group the sheets.
CTRL + a to select all cells on the active sheet
Change the font to 8. Select a single cell.
Ungroup the worksheets...............select any of the non-active sheet tabs
to clear the grouping.
Gord Dibben MS Excel MVP