110:4 One may not knead a quantity of dough that is larger than the size needed to take challah (refer back to 35:2). It is preferable to use less because if the dough is greater than the size needed for challah, it would be impossible to work it all at one time. (If one did work a larger quantity of dough, it is valid after the fact, even if one did so intentionally – Mishnah Brurah 456:7.) The result is that a part of it will sit without being worked and there is concern that it will leaven. If the dough is soft, one may not add flour to harden it.
110:5 We must not stuff the dough into the measure because of the possibility that it won’t knead properly and some flour will remain inside the matzah that will leaven in cooking. One must also be careful not to put the flour too close to the water so that none of it will fall into the water. Similarly, the one measuring out the flour must not go near the dough, nor near the water. Optimally, one should try not to put his hand on the flour unnecessarily because his hand will warm it slightly.