It says (Pesachim 32a), if one eats Terumas Chometz unintentionally on
Peach, one is obligated to pay. The reason the gemora explains in 32a is because one pays according to size/weight. Why do we not say that since we have to get rid of and burn the chametz, it has no shiur (size) as we say by lulav of idol worship since it must be burnt, it does not have a shiur (size). So even if one pays according to size/weight, why do we pay - there is no size?!
The principle that something which must be destroyed has no Shi'ur (Ketusi Michtas Shi'urei) shapplies only with regard to Halachos that require a Shi'ur to fulfill a Mitzvah, but not with regard to matters of reality (e.g. did he eat Terumah or did he not eat Terumah). As the words describe, something that must be burned is as if it is "chopped up into little pieces" of ash already. It nevertheless certainly exists. Compensating for Terumah is a matter of Metzi'us, reality; one who ate Terumah must pay for it, no matter what condition it was in, based on the market price.
Rabeinu Chaim ha'Levy discusses this. Tosfos in Eruvin (80b, DH Aval) alludes to it in his discussion about making a Lechi with an item which must be burned. Tosfos says that the principle that something which must be burned is lacking in its Shi'ur does not apply, because in order to be a Lechi it does not have to be one solid item, but it can be made of a line of crushed particles or ashes, placed one on top of the other until they join to make the proper height of a Lechi. The same applies to eating Terumah.
M. KORNFELD