If I cast enlarge on a creature and then attack, can my opponent cast Celestial Flare on my Enlarged creature, or do they actually HAVE to block it?

Pending that's the only creature you're attacking with, you would have to sacrifice it as a result of Celestial Flare, leaving no lasting effect from Enlarge and freeing them to block however they like.

However, as Celestial Flare targets a player, not a creature, and leaves it up to said targeted player to choose which attacking or blocking creature is sacked, odds are you're not just attacking with the Enlarged alone, meaning you could choose one of the other creatures to sacrifice and then they WOULD have to block the Enlarged creature.

Does that make sense?

Simply put, if they remove the creature you cast Enlarge on, then there's nothing there for them to be required to block and, as such, they can block normally (aka however they choose).

Thanks so much for your reply! This really helps.

Probably not super important for this scenario, but it's important to understand how it works.

I have a single creature, I declare an attack with it, after that, I cast Enlarge on it, while Enlarge is still on the stack, you cast Celestial Flare targeting me. Celestial Flare resolves and I'm forced to sacrifice my one attacking creature. Enlarge tries to resolve, but at this point it fizzles because all of its targets are no longer valid.

There's a problem with your example in the last post.

I have a single creature, I declare an attack with it, after that, I cast Enlarge on it, while Enlarge is still on the stack, you cast Celestial Flare targeting me. Celestial Flare resolves and I'm forced to sacrifice my one attacking creature. Enlarge tries to resolve, but at this point it fizzles because all of its targets are no longer valid.

You can't cast enlarge during combat it's a sorcery and can only be cast in your pre-combat/post combat main phases.

So you would have to cast enlarge pre combat, then attack, at which point celestial flare can be declared after attackers and you lose your dude.

Combat is 5 steps. Only instants or abilities can be activated/cast during combat. in a nut shell

1. Precombat - can cast instants and use abilities
2. Declare attackers- can cast instants or use abilities _after_ attackers have been declared and tapped
3. declare blockers - can cast instants or use abilities _after_ blockers have been declared and blocking/damage assignment order has been finished
4. damage - no abilities allowed, damage is assigned, any triggered abilities will go on the stack to be resolved in the clean up step
5. clean up - chance for spells and abilities after combat

Yup, you're right. I didn't look closely enough at it as I don't usually run with cards that pre-buff my creatures so I just assumed it was an instant.