The "optimal" kmer size length topic have been widely discussed across several posts. I would recommend you to do a quick search to get in contact to de novo transcriptome assembly issues.
Speaking about the Kmer size, generally, a good choice is to set it between
half to 2/3rd of the read length. A too small length will lead to high amount of short contigs, most of them partial length assemblies, while if you choose a longer size, will result in few long contigs.
Yo may need to perform several trial runs with different kmer lengths, and select the best one according to several statistics such as N50, total scaffold length... etc.