When printing merge data that consists of long text strings, the text may not fit on the label and will run off the edge.With merge printing, all printed labels are the same length, text boxes and the label length should be sized to so they are large enough for the longest text string in the merge data.

Also, selecting "Prioritize box size" for "Text box settings" will cause the printer to automatically reduces the size of text in the case of a long text string so it can fit.