QUESTION:
What is the largest object the HST has ever seen?

ANSWER from Bryan Miller on April 23, 1996:
I would have to say that the largest object ever looked at with HST is one 
of the giant elliptical galaxies like M87. M87 is a galaxy in the Virgo 
cluster of galaxies that is relatively close to our own galaxy, the Milky 
Way. While the Milky Way is about 100,000 light years across, giant 
elliptical galaxies like M87 can be several times bigger than this (maybe 
as big as 500,000 light years across). Remember that a light year is 
equivalent to about 6 trillion miles.

Now of course a galaxy isn't really a single object, because they're made 
up of billions of individual stars, so someone might argue that the largest 
single object ever observed with HST would have to be something like the 
Orion or Eagle nebulae, which are star forming regions in our own galaxy.