The optimal text size varies depending on what background is being used.
We suggest you use stronger fonts such as Arial, Verdana or Tahoma as opposed to serifed fonts (fonts with small tails contained on the end of characters) so they don't fade into coloured or transparent backgrounds. You should only use serifed fonts at larger sizes (14 point or above, preferably with bold) when dealing with coloured or transparent backgrounds.
10 point is fine on a white or very light coloured background, but if the background varies from this then we suggest as a minimum going to a 10 point bold or if space permits, then preferably 12 point.