Source suggestions for small spades

If the amp uses plain screw terminals you want #6 spades.

The receiving ends vary as to what gauge of wire they will accept, but they are still plentiful and inexpensive.

DeKay

Based on your inability to properly measure the post, you need to find a friend with a set of calipers and maybe some DIY skills to share.  It isn't hard to learn but maybe you need a little help.

You need to fit the lug to the terminal.  A too-big lug gives a poor connection, works loose, and can twist and short.  

But lugs that are the right size for your wire.

I much prefer crimped and soldered to the ones with little screws.

There are cheap flimsy and low current lugs available in kits at Harbor Freight, Amazon, ebay and other places.  Avoid those.

Consider a BFA style banana which is much more foolproof and IMHO gives a better connection.

Jerry

The OP has a late 60’s Kenwood receiver with screw speaker terminals. He can’t use bananas on the amp end.

I use connectors from Anticable...https://anticables.com/speaker-wires/2-channel

Minimalist designs, pure copper. They even have "vintage" pure copper spades which I believe to be what you are looking for.

Of course, looking at how those connectors are attached to the chassis, it does kind of make you want to make new one's with banana plugs, don't it? :D