A: Oh, what’s the hurry? I mean it’s only November.
You’ve got lots of time till Christmas.
B: But you know what Santa says: Post early for Christmas.
A: That’s right; have to do what Santa Clause says. Oh!
How are you going to get some of these larger parcels in the mailbox?
B: Oh, they won’t fit in the mailbox. I’ll have to take them to the post office.
What does “get” mean in this dialogue?
Does it mean “fit” or something else?
Thank you
Fit and get are not synonyms. But I think you understand what the sentence means.
To fit refers to size. Is the package small enough to fit inside the mail box?
To get in is the verb, to actually put the package in the mail box.
" How are you going to get some of these larger parcels in the mailbox?
Thank you so much, NearlyNapping 
How are you going to put some of these larger parcels in the mailbox? (The question is asked because the size of the packets is too large for the ‘mouth’ of the mailbox or it does not fit in the ‘opening’ of the ordinary mailbox. That’s why B suggests taking the parcels to the post office).
Thank you so much, Anglophile