Most cells are very small. What physical and metabolic  constraints
limit cells size? What problem would an enormous cells encounter? What
adapations might help a very large survive?

Cells need to be able to transport very small molecules(These could be
ions, Atp, amino acids, etc) at quick rates. It is also often
necessary to have consistent molecule concentration thoughout a cell. 
As a cell expands, volume becomes so great that passive means of
transport such as osmosis become inadequate. Even active means,
including ion "pumps" in the cell membrane, are not able to take
molecules over long distances. Cells can overcome this problem, but
only to a certain degree. A nueron, you may know, has a long extension
known as an axon. Nuerons have developed two different means of active
transportation in order to to bring molecules from the cell body to
the ends of the axons. Hope this helps.

In cell culture most non-neoplastic cells only grow to a certain point
(both in number and size)due to a phenomenon known as contact
inhibition. As cells come in contact with each other, growth stops due
to the molecular cues generated. The same occurs in vivo. Of course
apart from spatial constraints, hormonal and at times migratory
cues(as in the case of the developing neural system)also contribute,
not only to cell size but also in sculpting the actual morphology of
the cell.
The question about metabolic constraints is an excellent one and a bit
more theoritical. It will depend on the system you are talking about.
i.e. where are the cells in question? And what cell type are we
talking about? If they're in culture, quite obviously the nutrient
medium (although usuallly in much excess) may become a limiting factor
to growth. In vivo there are many more answers. in general, in
non-germ line cells, it is thought that cells grow untill they achieve
a ceratin cell size and beyond this they begin to undergo mitosis ( in
case these cells do that; I mean cells such as the neurons etc have
lost the ability to undergo mitotic divisions). Again some people
think this is an antiquated view- stressing that signalling molecules
guide cell size- you can google numerous references- e.g. The molecule
Akt controls cell size increase (hypertrophy) in cardiac cells.
Naturally the easiest questiosn to prehaps answer is the one about
problems that an unusally large cell will encounter- avaiability of
nutrients, ATP, ability to communicate with the environment
(surface-area/ volume issue- which may also very well apply also in
another area- absorption, in particular if the cell si adapted to
absorb) etc