That’s OK/all right/fine.

[diamondcutter]

My friend suggests a time to meet up. He says this.
How about this afternoon?

If I agree, could I simply say the following?
That’s OK/all right/fine.

 

[emsr2d2]

All three are possible, although "That's all right" is my least favourite. There are loads of possibilities, all of which could include "Yes" at the start but they don't need to.

That's fine.
That's OK.
That works for me.
Great.
Sure.
Fab.
Cool!

Of course, you could simply say "Yes".

Note that if your friend said "How about this afternoon?", he didn't suggest a time to meet up. After you agree to meet in the afternoon, you would then have to agree on an exact time.

 

[jutfrank]

I think by far the most likely response would be a simple OK. There's no need to say That's.