Is the chiyuv of the R'i'ya on every male over 13, so that each individual needs to bring one?
Also - is the chiyuv of 2 ma'ah kesef also on each male?
Finally, what is the shiur of achila rishona of the chagiga?
tuvya marcus, Jerusalem, Israel
Technically, every male over thirteen is obligated in Ri'iyah, as well as 2 Ma'ah Kesef. However, there could be reasons they would be exempt (i.e. according to Tosfos in Pesachim 3b that not owning land makes one exempt from Ri'iyah).
Regarding your second question, there is an argument what we Pasken regarding Achilah Rishonah. The Meiri in Beis ha'Bechirah (7b) says that we Pasken like Rebbi Yochanan, that as long as one has the value of a small sheep from Chulin, he may add Ma'aser money to bring his Korban, even if it ends up being a big ox. However, the Meiri notes that this is the subject of an argument among the Rishonim.
All the best,
Yaakov Montrose