How To Filter Multi Fasta By Length??
http://itrylinux.com/use-awk-to-filter-fasta-file-by-minimum-sequence-length/

non-awk solution:

seqkit seq -m 120 test.fa -o test.fitlered.fa

download seqkit from here: https://bioinf.shenwei.me/seqkit/download/

Here's an awk-based approach, if your FASTA records are single-lined (header on one line, sequence on the next line, and so on):

$ awk '!/^>/ { next } { getline seq } length(seq) >= 120 { print $0 "\n" seq }' input.fa > output.fa

If multi-lined (header on one line, sequence split across two or more lines), then the input.fa would need preprocessing to make it single-lined. There are other answers on Biostars that explain how to do this.

Via: http://itrylinux.com/use-awk-to-filter-fasta-file-by-minimum-sequence-length/