A kizayis is approximately 34 grams if you want the largest shiur (1.2 oz).
I have a related question. Since challa is one of the matanos that can be eaten b’tuma and b’gvulin, why don’t we give challa to kohanim in Israel? I know that people consider most kohanim today to be of safek yichus, but we have plenty of m’yuchasim as well.