Quickly take note that the size is out (5)
I believe the answer is:
seize
'quickly take note that' is the definition.
I can't judge whether this definition defines the answer.
'size is out' is the wordplay.
I cannot really understand how this works, but
an anagram of 'size' is 'seiz' which is within the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for seize that I've seen before include "Grasp hold of" , "square in Paris" , "Impound" , "Grab suddenly" , "Snatch" .)