If the size of the contents in a cell are too big to fit then they will either a) wrap to the next line (see home->wrap text), b) overflow into the next cell (but only if that next cell is empty) or c) get clipped (if wrap text isn't on and the next cell isn't empty) If this isn't the issue you can get odd formatting issue when copy and paste from another document. Try clear formatting (home->Clear and in the drop down select clear formatting)
