Is the spade size an issue sonically if it’s larger than the contact area ?

No, it is not an issue, don't worry about it.

It is not the contact area on the U that matters but the flat part against the metal doughnuts that grip it that provide the majority of the contact area.


A bigger issue is making sure your spade is soft. Hard spades or rhodium coated spades are too hard to grip when tightened and can easily come loose.  Some of the best IMHO are the spring-loaded WBT types.

Hard surfaces are a pain. A little dab of nickel Never Seez, will really help though. Just a tiny bit on a Q-Tip. torque, back off, retorque tighter, back off, TORQUE slowly. If you get it tight (with lube) it will stay tight, and NNS is a great contact enhancer. Nickel, copper, lead, tin, etc.. It's all in Nuclear Never Seez.. 

OP think of the fuse in your amp.. Think how THIN that filament is.
Now think of the contact surface on your speaker cables... TIGHT is good.. NO mater the surface area, Tight is good...

Regards

They really spell it Nuclear Never Seez? Wow. 

In the past I have removed the spades and replaced with smaller spades that fit my amps better. I'm more anal than most and a half decent solderer.