Question

I am wondering what can be the maximum value that a <input type="hidden"> can store, I am not taking about the maxlength attribute, but the string which it can store.

Answer 1

http://www.w3.org/TR/html401/sgml/sgmldecl.html

 QUANTITY SGMLREF
                  ATTCNT   60      -- increased --
                  ATTSPLEN 65536   -- These are the largest values --
                  LITLEN   65536   -- permitted in the declaration --
                  NAMELEN  65536   -- Avoid fixed limits in actual --
                  PILEN    65536   -- implementations of HTML UA's --
                  TAGLVL   100
                  TAGLEN   65536
                  GRPGTCNT 150
                  GRPCNT   64

edit

according to html 5:

The algorithm described below places no limit on the depth of the DOM
tree generated, or on the length of tag names, attribute names,
attribute values, text nodes, etc. While implementors are encouraged
to avoid arbitrary limits, it is recognized that practical concerns
will likely force user agents to impose nesting depth constraints.