What size tires should be on a Ford Festiva?

145sr12 is recommended. Realistically, 145 or 155 or 165 in 12's will work (I'm running 155's now).

You could also upgrade to 13" rims on a Festiva, with a stock suspension (check the junkyard for Mazda wheels which are hubcentric to make it easier). Hubcentric means that not only will the lug bolt pattern be identical, but the center hole will be the same size as well. Of course, this would cause the speedo to be off, but the speedo gear from the transmission can be replaced as well if that's a concern.