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.
Is there a hard line between a small sword and a dagger?
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!