Source suggestions for small spades

Parts Express

Just looked at the back of those.  Yes, Parts Express is decent but so is Amazon.  Honestly found the PE connectors for whatever reason never keep a crimp well enough.  I've thrown out many bags from them.  You'll want to match the diameter of your wire AND you definitely want to use an insulated spade.  The color on the insulation determines the size wire it is designed for.  The insulation will minimize the chances of your amp shorting when you move a cable or the receiver.

Absolutely recommend a ratcheting crimp tool specifically for insulated connectors.

You may also consider a ring terminal instead of a spade in this case, if the screws come all the way out. 

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