thanks for the suggestions
the mod does have a prune option but that removes everything from the chat and i want to keep some of it

i want to delete only specific rows from that table, basically just the last 2/3 of the table, by using an sql command
the size of the is around db if 80MB which is 1/3 of the total so i was thinking maybe i can reduce it to improve speed

Why in the world is this 1/3 the size of your database? Inferno shoutbox... isn't that the one only available on nulled type sites? I could be wrong but using mods from sites like that can leave it/you open to exploits, perhaps there are just that many users in the shoutbox but are there that same number of users on the actual forum? If not look to see if the inferno shoutbox is being exploited somehow and fix if you intend to keep... if that is the case though I'd simply remove it, best to not run mods from nulled/cracked sites.

In one word: NO.

Besides, even tough reducing the DB size is always good, the main issue is your shoutbox plugin, you're better off with it so I strongly advise you to uninstall it, anyone with minimal knowledge could get into your site and dump the DB with a simple exploit if you use a nulled or abandoned product.

some logging option was on by default and maybe that explains the size. all my mods are from vb.org
Inferno Shoutbox was available a long time ago on vBulletin.org: https://vborg.vbsupport.ru/showthread.php?t=147346

Either way the last time the shoutbox was updated was in 2008, 9 years ago! That's not safe at all, specially when it comes with BB's when a lot of exploits tend to be published with manuals.

I would suggest DbTech's vBShout which is working for 3.8