How does Spellheart Chimera count Fuse or Split cards?
If "Far // Away" was the only card in your graveyard, Spellheart Chimera would be 1/3

Split cards always count as a single card: 


  708.4. Although split cards have two castable halves, each split card is only one card. For example, a player who has drawn or discarded a split card has drawn or discarded one card, not two.


Note that the Chimera asks for instant or sorcery cards, an important difference to just instant/sorcery. The latter only exists on the stack.

It would also count as a single card if the two halves had different types, for example if one half was instant and the other a sorcery.