Assuming that your sequences are on one line:

​$ awk '{ \
    if ($0 ~ /^>/) { \
        header = $0; \
    } \
    else { \
        l = length($0); \
        if ((l >= 15) && (l <= 30)) { \
            printf("%s\n%s\n", header, $0); \
        } \
    } \
}' foo.fa

Look at this Biostars question if you need to preprocess your FASTA file to put its sequences on one line.