There is much contradictory info on this, but bar shatya’s is what wikipedia says. Other sites say 65 pounds-130 for a yearling lamb. But that’s with modern pproduction methods. Ancient animals were much scrawnier.
But it’s hard to get good info, as lamb is not that popular…it’s the silence of the lamb weights.
You also have to remember that the ancient kezayis was much smaller than today’s halachic kezayis, more akin to an actual zayis. So that works in pba’s favor.
(kezayis today fallows halachic mesorah, so one who eats a kezayis of matzah based on the old standard isn’t yotzei- sorry Rabbi Slifkin…)