If any of the creature cards you exile has a characteristic-defining ability that defines its power and/or toughness, that ability will apply. For example, if Dungrove Elder is exiled this way, its power and toughness while it's in exile are equal to the number of Forests you control, and Sutured Ghoul's power and toughness will change as the number of Forests you control changes. If the characteristic-defining ability can't be applied (for instance, it relies on a choice made as the card enters), then use 0.