No one actually knows how to lculate what speaker cable they need

May I ask from who and what did you order? Fully assembled? Can you provide a link and how they will be terminated? Thanks.

I ordered them from chameleoncables on ebay:

2 of these: https://www.ebay.co.uk/itm/192412561182
And 1 of these: https://www.ebay.co.uk/itm/192044378960

I spoke with the shop owner and he's going to cut the cable into 4 x 2.5 m lengths and terminate them so I have four black ends and four red ends.

I might need to use a reducer or a termination block to connect the binding posts, but I have lots of room so it might fit directly.

Nice choices.  Let us know.  I saw those same spades and they should works nicely. If to big or bulky to fit, then you can buy spade to banana high quality connectors.  

Do you have a link to those adaptors? I don’t think I came across any.
Nevermind actually, I googled. I might already have some bananas that’ll work.

My plan is to bend the spades back by 45 degrees to give myself additional clearance if needed. it’ll be interesting.

Cardas and other quality companies make them.  Here is a very nice one from Cardas. 

http://cardas.com/cab.php