With some training, I'm sure your users will be able to accept that the amount of data they can type into a cell will be limited.
There is no way to set the number of characters allowed in a restricted document. (For that you need an ordinary form, and a macro that takes care of the spellchecking. See
http://word.mvps.org/faqs/macrosvba/...ProtectDoc.htm.)
In a restricted document, you should be able to spellcheck, though. Are you saying that this doesn't work for you?