According to Sephardim, if one ate less than a kezayit of bread there's no obligation to wash, if one ate one kezayit one should wash without a Bracha of Al Netilat Yadayim and if one ate a Kebaytzah (2 Kezaytim) one should wash with a Bracha. (S"A 158:2, Yalkut Yosef 158:4). According to Ashkenazim preferably one should have a Kebaytzah to wash with a Bracha but if one needs to have a kezayit one should wash with a Bracha. If one eats less than a kezayit one should wash without a Bracha. (Vezot HaBracha chap 2, pg 13 based on Igrot Moshe 4:44 and Rav Elyashiv)
If one is about to commence a meal with bread and is not sure how much bread will be eaten should one skip the brach al netilat yadayim? Or is the additional food counted towards the kebeya?