Spade connection size for McIntosh MC2155 Amp

I think your amp has a barrier strip for speaker cable connectors. It probably will only accept a small spade, probably 1/4 inch. The 1/4 in are very nice, though. I prefer them myself. I would call McIntosh and find out, for sure, if it is the correct size.

As a longtime owner of a vintage Mac 2105 and a 2100, consider having a capable technician replace the old terminal strips with good quality modern posts that will accomodate spades or bananas. Yes, you will sacrifice the choice of 4,8 or 16 ohm taps, and will have to choose one of them, but it's worth it. Even if you can get the correct spades, connecting them with typically thicker, stiffer modern cables is a real pain. The small screws don't hold well, and sometimes just let go.
I made the change and love it. Sound quality was improved, and no more hassles. In my case, I always end up with 8 ohm speakers anyway.

I think it all depends on the quality of the barrier strip. Some of my gear has them and I wouldn't trade them for anything. If, like Roxy54 says, you don't have good ones, it may be a good idea to replace them. I don't see why you would have to sacrifice the extra taps either. All of my tube amps have regular binding posts for all the taps.