No, it's not a stupid question. :o)
It's kind've off-topic to the entry itself though and/or the community (about writing software programs - 'clients', that people can download+use to update their journals; or otherwise interface with the LJ site), which the entry's in though. You'd probably generally have more luck with getting an answer if you tried a community search
to locate communities that discuss HTML, to ask there about the markup code needed for a scroll bar/whatever. Otherwise you're reliant on the original poster a) receiving comment emails and b) deciding to reply, assuming c) they know the answer; or, someone else reading the older entry and replying to your comment.
Generally, you can also view page source in your browser, to find out 'how did they do that?'. Because the poster of this entry had a HTML error in their entry the scrollbar wasn't deliberate on their part... so that wouldn't be that helpful. A window with a scroll-bar is a pretty common sight on userinfo pages though, so you could view source at a page like that to see the code. HTML tutorials on the web e.g. lissaexplains.com are helpful too.
Since I'm commenting already... and I know I wondered how people did those scrolling textboxes for ages :)
...you can do what you want using the textarea
<textarea rows="4" cols="20">
line 2 etc.