As for giant versions, eyeing the raws I remember seeing that some giant animals usually remove and add a few tags to the regular version, and modify the grazer to a smaller number...
for the hare, giant hare and hare man, seem to be [GRAZER:17143], since they don't get changed for some reason... check the panda and capybara they do get changed in their giant versions...
As for the pasture size,
read this threadThat's the grazing self-sufficiency thread people are talking about here, the SCIENCE!! was made there, I even came up with a table for the recommended pasture sizes that so far has worked pretty well.
I did notice that in DF2012 the grazer numbers were increased, so we can get away with smaller pastures now...
The math is quite clear now, but based on empirical data, basically a forum member, Jurph, started experimenting with a small grazer, goats to be exact, and made several pastures of different sizes, starting from 1x1 and going up, 1x2, 1x3, 2x2, 1x5, 3x2...
Placed the goats there and waited to see if they would deplete the pasture and go hungry, if they did the size was too small, and keep experimenting with bigger pastures until finding the sweet spot.
I reproduced the experiment and arrived at the same conclusion, that a goat (grazer:1200) needs 16 tiles of grass to survive.
From that number I then extrapolated the data for the other grazers. It seems to pay of to keep the pastures as close to a square as possible, to avoid weird pathing behaviour.
I also came up with another math, based on steps, time units, pathing behaviour but didn't get very far since the grass growth was a needed but unknown variable... I still have the spreadsheet with the table and calculations if you're interested.