If you have a PC, try holding down the CTRL key then tapping + or - (plus or minus) to make things bigger or smaller. If you have an Apple Mac, use the Command key instead of CTRL. For both PCs and Apple Macs, if you have a mouse wheel, you can use that instead of + and -, which is a bit quicker.
For more help, see the BBC's excellent, albeit archived, My Web My Way help guide.