Word interop problem : String parameter too long.

OleVarient (type used to pass your parameters to COM server of MS Word) has length limit of 255 charaters. To wordaround your case, you can try the following method:

searchAndReplaceLongTextInStory(wordApp.Selection, strSearch, strReplace);
/*********************************/
        private void searchAndReplaceLongTextInStory(Word.Range rngStory, string strSearch, string strReplace, Word.WdFindWrap wrap = Word.WdFindWrap.wdFindStop)
        {
            List<string> subs = new List<string>();
            int counter = 0;
            while (counter <= strSearch.Length)
            {
                if (strSearch.Length < counter + 250)
                {
                    subs.Add(strSearch.Substring(counter, strSearch.Length - counter));
                }
                else
                {
                    subs.Add(strSearch.Substring(counter, 250) + "#r#");
                }
                counter += 250;
            }
            rngStory.Find.ClearFormatting();
            rngStory.Find.Replacement.ClearFormatting();
            rngStory.Find.Text = strSearch;
            rngStory.Find.Wrap = wrap;
            rngStory.Find.Replacement.Text = subs[0];
            rngStory.Find.Execute(Forward: true, Replace: Word.WdReplace.wdReplaceAll);
            rngStory.Find.Text = "#r#";
            for (int i = 1; i < subs.Count; i++)
            {
                rngStory.Find.Replacement.Text = subs[i];
                rngStory.Find.Execute(Forward: true, Replace: Word.WdReplace.wdReplaceAll);
            }
        }

Note that I just coded it from my mind modifying other's example, you may have to debug before using it, but you get the idea.