I only found the number 150 bp as an answer. But either both fragments are 150 bp long (unprobable) or there must be another lenght inbetween. I found the paper of Ioos et al. 2009 (Fusarium circinatum) to be one of the first papers to mention this method, but I couldn't find the length of the shorter fragment. In Francois et al. 2007 (Bursaphelenchus xylophilus) I learned how this method works and the reason why I want to know this: I just don't want to run another qPCR without this internal control because I want to have a gel-electrophoresis with a clear result :)