Longsword uses your STR to hit, not DEX.
Ahh I see my mistake now, thanks for clearing that up.
Larian have chosen to represent the historical longsword, rather than the one-handed arming sword which has been a staple of (A)D&D for so long.
The traditional longsword is, indeed, primarily a two-handed weapon. It was developed to be used in two hands and only used one-handed when mounted or in some other situation where using two hands was impossible. It should be around 4 feet in length.
The sword in that illustration does seems to be over-sized, however. That maybe due to the elf being quite short. The screen-shots of longswords carried by humans seem more in proportion, assuming human men in BG3 to be around 5' 8" or so.
Sword definition is a nebulous business, but antiquarians and historians tend to make a distinction between longswords and arming (or 'broad') swords.
It looks a fair bit bigger compared to this:
https://gizmodo.com/this-stunning-medieval-longsword-was-just-pulled-from-a-1796300991I think it looks out of proportion in the game.
