What is the damage type associated with the goblin playable race's Fury of the Small trait from Volo's Guide to Monsters (p. 119) if the associated attack or spell deals multiple types of damage, such as with the flame strike spell?
The description of the goblin playable race's Fury of the Small trait (VGtM, p. 119) says:
When you damage a creature with an attack or a spell and the creature’s size is larger than yours, you can cause the attack or spell to deal extra damage to the creature. The extra damage equals your level.
It doesn't specify a damage type, so it's reliant on the damage type of the associated spell or attack that the extra damage is tied to. However, there are no official rules on how to determine the damage type when the associated spell/attack does multiple types of damage, such as the flame strike spell (which does both fire and radiant damage).
As of the release of the 2019 Sage Advice Compendium, there has been no official ruling on this or similar cases. However, official rules designer Jeremy Crawford did provide unofficial guidance on Twitter regarding the hunter's mark spell, which also does extra damage of an unspecified type. He said about the damage type of hunter's mark:
Hunter's mark uses the same damage type as the attack that triggers it. If the attack has more than one damage type, choose one.
He further clarified, in response to a misconception by the other user:
All damage has a type.
Thus, it's clear that Fury of the Small must be associated with some damage type, and he unofficially ruled that when there are multiple damage types associated with the extra damage of hunter's mark, the player can choose which damage type the extra damage is. It seems likely that other abilities like Fury of the Small that cause extra damage of an unspecified type associated with an attack or spell would work the same way.
It's up to the DM and player
Unfortunately, there is no specific rules direction as to how handle cases where you can add damage to an attack and what to do if there are multiple damage types.
While the other answers provide Jeremy Crawford's thoughts, as well as this question, that's all we really have to go on.
However, given that the damage type is unspecified, and your little goblin is dropping a SR resource to do this, I think it's reasonable to allow the player to pick which damage type they'd like to apply to their Fury.
In general, this will only come up when dealing with a monster who has resistance, immunity, or vulnerability to a delivered damage type. But even in those cases, it generally make sense to give the player the feel good moment and let them unleash their fury in the most thematic or most impactful way possible.
Quote from Volo's:
Fury of the Small. When you damage a creature with an attack or a spell and the creature's size is larger than yours, you can cause the attack or spell to deal extra damage to the creature. The extra damage equals your level. Once you use this trait, you can't use it again until you finish a short or long rest.
Interesting, the ability specifies the amount of the extra damage but not the type. Common sense dictates the type is the same damage as the source of the original damage unless sage advice or errata says otherwise. Such as fire from a fire cantrip spell or piercing for a piercing weapon. This might allow the caster/attacking goblin to decide which type of damage is granted if there are two or more damage types involved.
Sage advice notes that sneak attacks have the same damage type as the weapon that caused it, which can be used as guideline. Sadly this doesn't answer the question about what damage bonus types spells get which aren't using weapons, and have 2 or more damage types, like Flame Strike.