How to mark sequences one line above already marked sequences

Im currently working with a FASTA file containing thousands of nucleotide sequences coupled with an identifier above each said sequence.
I need to select and bookmark sequences greater than a particular length AND the identifying sequence which lies directly above it. How would I go about doing this?
I have been using ^.{300,} to mark the sequences which I need, but I don’t know how to also automatically mark the identifiers above them. Any help would be greatly appreciated! Thank you.

@LionelWaschnig Does this help?
.+\R.{300,}

Perfect  Thank you!