that's the biggest cake

[Maybo]

Am I using correct tenses in "That's the biggest cake I've ever seen"? May had never seen a cake bigger than the one in her birthday when she had the following conversation.

John: How was you birthday going yesterday?
May: Great! My mother bought me a cake. That's the biggest cake I've ever seen.

 

[GoesStation]

John: How was your birthday party [STRIKE]going[/STRIKE] yesterday?
May: Great! My mother bought me a cake. [STRIKE]That's[/STRIKE] It was the biggest cake I've ever seen.

The party, and seeing the cake, happened in the past.

 

[Maybo]

[STRIKE]That's[/STRIKE] It was the biggest cake I've ever seen.

Is "that" incorrect because it sounds like I'm pointing to the cake?

 

[GoesStation]

Is "that" incorrect because it sounds like I'm pointing to the cake?

Maybe. It's not really wrong, though. You could be mentally "pointing" at it.

 

[Tdol]

May had never seen a cake bigger than the one in her birthday

I would use on/for her birthday.