What size spade for Vandersteen 3A?

You are correct that you will need smaller, 1/4-inch spades. I have a pair of Vandy 3A Sig's, and have used small spade lugs made by MonsterCable with them. The Alpha-Core Goertz speaker cables that I am currently using are terminated with Alpha-Core's small silver spades (they offer a larger and a smaller size - I have the large spades on the amplifier end of the cable, and the smaller size on the speaker end). As I recall, Alpha-Core also sells a very good rhodium spade (again, in the regular and "small" size).

Cardas 7mm GRS spades should fit. That is what I am using on my Model 5. I am not sure about CCMS-S.

Kimber's standard spade (not the post master) fits nicely on my 3A Signature's. I don't think there is a difference in the barrier strips between 3A and Signature.