In order to change all fonts you could try this:
1) Open your spreadsheet
2) Make sure all sheets are visible (not hidden)
3) Right Click on a Tab (sheet) and then Select All Sheets
4) Left Click the little square in the Top Left (to the Left of A and above Row 1
5) Select whatever Font and Size you want.
6) Right Click on a Tab again and select Ungroup Sheets
NOTE: EVERY Cell on EVERY Sheet will now have the same Font and Size
Hope this helps.
Bye 4 Now,