Hi,
Create or edit conf/userstyle.css, place this code in the file :
body div.dokuwiki div.syntaxhighlighter {
max-height: 3cm;
overflow-y: scroll !important;
}
Choose the max height that you need, here 3 cm is very small. You may use any css supported unit. See
https://developer.mozilla.org/en-US/docs/Web/CSS/length . Maybe
20lh is what you need.