For THE HASSASSIN’S THRUST what makes a weapon small? If it is Light, would it be considered small?

For THE HASSASSIN’S THRUST what makes a weapon small? If it is Light, would it be considered small?

Hullo, Charlie D,
No, I don't believe that if an item or weapon is Light that it is necessarily small, though there's some logical benefit to that.
For my own purposes, and I hope the game designers and rules people agree with me, an item that is small is one that fits comfortably in the hand.  Whether you consider Light items and weapons to be in that category, is up to you, but I prefer to take the sandbox approach to this and make that decision on a case-by-case basis.
Hope that helps.

Is this in the core book or something else?

core book

Like JonnK, I'd just wing it by thinking, any weapon that could easily be concealed in one hand/ sleeve: knife, garrote, scissors, ice pick, glass shard, rock, iPod...