Smallsword blade lengths would typically be 27 to 33 inches (the photo here just shows the hilt, there's a lot more blade extending beyond the lower margin), while a dagger's unlikely to go past 15-16 inches. Daggers would also rarely have these styles of shell-guard hilts.
However, I'm not sure if you might actually be asking for the distinction between shortswords and daggers - smallsword isn't a general term for a sword with a short blade, it's a specific category denoting a type of sword descended from the rapier, but optimized for convenience of carry for fancy-lad aristocrats (hence a short blade compared to proper rapiers, and a much simpler hilt, so you can wear it around court without it constantly banging on any tables or doorframes you pass by).
In that case, and indeed more generally with historical weapons, there aren't a lot of hard lines, and most terminology is going to be modern historians/collectors retroactively trying to categorize things which historical people would have just called the word for "sword" in their language (maybe with an adjective if they were feeling real fancy). One type of weapon that really shows the blurred lines here is the kindjal, which is associated with the Caucasus, and is typically considered a dagger, except examples like this one are long enough to seem almost gladius-like.
Oh yeah I was for sure mixing up smallsword and shortsword, thanks for clarifying!
Makes me wonder why full hand guards like that didn’t evolve earlier. They are clearly better at protecting the hands than a simple cross guard and they definitely had the technology to make them
There were actually some ancient swords with hand protection, like the Greek kopis. But most definitely has pretty low protection. Part of it has to do with the style of fighting - swords with little hand protection were typically used alongside shields, and so you protected your hand by maneuvering the shield to intercept an opponent's blade. The development of complex hilts in Europe happened as shields were becoming less prominent due to armor, so it was likely influenced by that.
Additionally, the extra hand protection on complex hilts has its costs, like extra mass and a different balance. There's a scholagladiatoria video talking about this. Smallswords like these ones actually represent this particular trade-off - they have much less protection compared to earlier rapiers, especially against cuts, specifically because those massive hilts were a lot less convenient to carry.
