sed
solução:
sed -E 's/(.*-)([0-9]+)(;.*)/;size=;/' file
A saída:
>97977-100;sample=Samp1;size=100;
TAATGATGATTTGT
>97978-60;sample=Samp2;size=60;
AACATTCAACGCGGTCGGTGAGTA
>97979-30;sample=Samp3;size=30;
AACCGTAGGAGTTGATGTGCGGT
>97980-20;sample=Samp4;size=20;
Ou com awk
:
awk -F'-' '/^>/{ $0=$0";size=" int($2) ";" }1' file