Building STAR Genome Index for nanopore RNA sequencing
The parameter is used to determine how much sequence STAR indexes on each side of a splice junction to improve its alignment accuracy.  For very long reads, this may not be ideal. I am not sure if STAR is capable of including multiple splice junctions since a long read is more than likely to span more than one.  

It may be worthwhile to consider aligning your reads directly to transcripts and taking the reads that align poorly and using them to look for novel transcripts.