What effects does being Silhouette 0 have exactly? There are several species in the Star Wars games that are Silhouette 0 and many species in various universes that would likely also be Silhouette 0 (ex: Hobbits). But what does that really mean? What benefits/drawbacks are there to being Silhouette 0?

The only strict mechanical difference is when it comes to combat checks. If your attacker is at least 2 silhouette larger than you, they add a difficulty die to their combat check.

If you are at least 2 silhouette smaller than your target, you remove a difficulty die to your combat check.

So if you plan to fight a wampa, your sil 0 jawa buddy is harder to hit but can hit that wampa easier than you.

If you're all up against a rancor, everyone benefits from the silhouette difference, because it would be silhouette 3 vs silhouette 1's & a silhouette 0 with the jawa buddy.

Beyond that, it's up to the GM how much being silhouette 0 affects the character. It is within the GM's right to say that a silhouette 0 character has additional difficulty when making certain physical checks but that's not a hard set rule. Personally I wouldn't place additional difficulty on physical checks as a rule but I would do it in fringe cases. Conversely, that character might have an easier time sneaking about or climbing into ventilation ductwork.

Edit: while I used star wars terms for creatures because I mistook this for the star wars forums, the rules apply the same in both systems regarding silhouette.