I'm sure there is a maximum size for a string  variable, but I've never hit
it, nor does a quick Google search point me to it. Suffice to say, it is
bigger than you're likely to need, especially if you're storing information
typed in by a user.

Message boxes also. Not sure there's an actual limit, but obviously the
screen size would impose a practical limitation - particularly if you have
line feeds included in the text.

Just do it - I doubt that you'll experience limitations.

Note, however, that there is a very real limit to the amount of text that
can be displayed reliably in a wrapped cell, so keep the individual cells
reasonably short.