What is the approximate weight / size for a kezayit of machine matzah / hand matzah?
Whatever size you give me I should multiply it by 2 since you need 2 kezayit, shouldn’t I?
For the actual mitzvah of eating matzah, one should be stringent and eat the larger, Chazon Ish (based on Noda Biyhuda) shiur, which comes out to two thirds of a machine matzah (or a little more than this). The Steipler wrote that a whole machine matzah is “easily” a Chazon Ish kezayis (Shiurei Mitzvos 25).
One does not need to eat two of these shiurim, for with regard to the stringency of eating two zeisim, one may rely on the smaller shiur. Therefore, eating a little more that two thirds of a machine matzah is certainly sufficient.
For Korech, a little over one third of a machine matzah is enough.
Note that the stringency of eating two zeisim applies specifically to those who have three matzos in front of them (usually, the leader of the Seder), and not to other participants. Yet, many have the custom for all to eat two zeisim. As mentioned, the larger shiur is sufficient to be considered two zeisim of the smaller shiur.
See also [Hebrew]: http://www.daat.ac.il/daat/toshba/halacha/berur-2.htm.