Question: I baked challos for Shabbos though forgot to separate challa before baking them. Can I take challa on Shabbos?
Answer: The Gemara (Eruvin 83a) writes that one only needs to separate challa when making a dough that is the size of a tenth of an eifa. Thus, the Shulchan Aruch (YD 324:1) writes that one making a dough from the five grains with a volume of 43 (and one fifth) eggs, must separate challa. As there is a machlokes as to the size of kabeitza nowadays, there are different customs as to how large the dough must be. Common practice is to separate challa without saying a beracha if using 2.5lb of flour (See Shiurei Torah p158) and with a beracha only when using over 5lb of flour.
The Shulchan Aruch (YD 327:2; 5) writes that ideally one should separate challa after kneading the dough, though if one didn’t do so then, they can still do so after baking it. One mustn’t eat the bread until they have separated challa.
The Mishna Berura (339:26) writes that one can’t separate challa on Shabbos because of tikkun mana (the finishing act). Thus, if one forgot to separate challa in Eretz Yisrael, they wouldn’t be able to eat it until they had separated challa after Shabbos (See Bechoros 27a; Shulchan Aruch YD 323:1).
The Chayei Adam (44:22) and Mishna Berura (261:4), however, write, that one who forgot to take challa in chutz la’aretz, may leave over a slice on Shabbos, eat the rest and separate the challa from that slice after Shabbos.In conclusion, if you leave a slice from one of your challos aside to separate challa from after Shabbos, you can eat the rest on Shabbos.