Why can't I use them?
So I was perusing some tumblr site for learning Korean earlier and came accross a two-vocabs note.

커요
big

작아요
small

And at the bottom of the note was written:

(don’t use this when you’re ordering food and drink)

Why is that? What's wrong with the words?

Thank you in advance for your help

Question

Why can't I use them?
So I was perusing some tumblr site for learning Korean earlier and came accross a two-vocabs note.

커요 
big

작아요 
small

And at the bottom of the note was written:

(don’t use this when you’re ordering food and drink)

Why is that? What's wrong with the words?

Thank you in advance for your help

Answer

Both mean:
It's (too) big.
It's (too) small.

And it doesn't make sense if you want to order something big or small.

If you want to order a big or small one:
큰거요: the big(ger) one please.
작은거요: the small(er) one please.