I'm not a Delaware nor Leghorn expert, so these are just guesses..... but that one does look like the Delaware chicks I have seen pics of. They are such pretty hens, I hope that's what she is!
On the chick size thing...... I am going to guess that chick size does not alway correspond to adult size. For instance, of our current Meyer chicks, the Welsummers are just massively bigger than the EEs and RIR. Yet RIR hens are said to be 6#, and Welsummers the same size.
On the other hand, White Plymouth Rock hens are 7.5 #. So by size alone that would be my guess. If that's the case, though, I think its a cockerel. That is a mighty large comb for such a pullet that young if its a Plymough Rock. It was the comb size alone that made me think Leghorn...... So, I'll revise my guess on #3 to:
White Plymouth Rock cockerel (now thinking this is more likely)
or
Leghorn pullet.
Remember though these are just guesses. Here are the breeds io have actual experience with as chicks:
RIR
EE
Welsummer
SLW
Australorp
Red/Gold sex link
Barred plymouth Rock