I don't know if its something from my side but cooper and gold chests have the same storage space of 5 rows and upgrading cooper to iron actually lower space by a row

Sounds like your pack changes number of slots and they haven't changed copper chest from its default when it was released

Is there a way to reconfigure it myself? Its a part of a modpack called SteamPunk [LPS]. I tried changing it ingame through the mod tab but i wasn't able to

It's in standard server configs so just look for sophisticatedstorage-server.toml in your world folder or in defaultconfigs folder (that will apply to any new world you create)