For split_libraries_fastq.py, the minimum read length is represented as a fraction of the total read length, rather than an absolute number: see the documentation for the min_per_read_length_fraction switch at
http://qiime.org/scripts/split_libraries_fastq.html . The default is 0.75 of the read length. Note that this approach anticipates that all reads will have the same length, so it just takes the length of the first read in the file as the read length.