Question: Is there any room for leniency regarding shi’urim on the second Seder night?
Answer: The Remo (Orach Chayim Siman 481) writes that all the dinim of the first Seder night apply to the second Seder night.
[An interesting exception to this is found in the Be’er Heitev (s.k. 3), who writes that on the second Seder night it is permissible to drink other drinks after the four kosos.]
Regarding shi’urim, there are two main opinions in the Poskim regarding shi’urim, the smaller shi’ur is known as “Rav Chaim Noe shi’ur” and the larger shi’ur is known as “Chazon Ish shi’ur”. Many endeavour to eat a larger shi’ur in a situation of a mitzvoh d’orayso, such as the k’zayis of matzo on the first Seder night, in order to fulfil both opinions.
Regarding shi’urim on the second seder night (where even the obligation of eating matzoh is midrabonon – because Yom Tov sheni itself is midrabonon), Rav Shlomo Zalman Aurbach (quoted in the sefer Yom Tov Sheni Kehilchoso page 76) maintained that one can be lenient and follow the smaller opinion (even for the k’zayis of matzo).