For doing M/S on acoustic guitar is it really essential that the mid mic be small diaphragm for the best outcome - or can it be large -D - perhaps for a certain sound that might even be better -I don't know. Of course I could just go and try it - but wanted to get some opinions going... thanks ! 

I think generalizations about LDC and SDC are inspired by specific mics that have been historically popular and were widely used. If you consider the wide scope of choices available today in the LDC and SDC format it seems difficult to substantiate a generalized preference for LDC or SDC as a form factor.
 
You'll learn more by just trying what you have available.
 
best regards,
mike
 
 
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... what Mike said.
 
Just make sure the mid mic is switched to cardioid.
 
I've done quite a bit of experimenting with mic combinations in front of a drum set and had good results using a 414 as mid switched to cardioid.

It shouldn't make any difference as far as M/S goes whether you use a small or large diaphragm. The convention of using SDCs is for two reasons. First, its smaller size may make it easier to get the two diaphragms closer together. Second, M/S is most-often used for acoustic guitars, and the small diaphragm is better for capturing the high-frequency subtleties of an acoustic guitar.
 

I've used both SDC and LDC microphones in M/S arrays. It really has more to do - I think - with the specific microphone and application.
 
I'd also point out that while convention suggests a cardiod pattern for the mid microphone, and in fact the math works better that way too, don't be afraid to try an omni as the mid microphone. It's a very different sound, but sometimes it works well.
 
You can also substitute a bi-directional microphone as the mid, but then it is (I  believe) more properly referred to as Blumlein...

A Blumlein pair is normally set up as two figure of 8's configured so that they are positioned 90° from each other.
 
*Wiki... "the microphone capsules are placed as close to each other as physically possible, generally with one centered directly above the other. The array is oriented so that the line bisecting the angle between the two microphones points towards the sound source to be recorded (see diagram). The pickup patterns of the pair, combined with their positioning, delivers a high degree of stereo separation in the source signal as well as the room ambiance."

Blumlein predicted the useful possibilities of coincident mic placement in a more ways than just the specific +45°/-45° fig8 technique that his name became associated with posthumously.
 
Also from WIKI:
 
"Mid-Side Stereo
This coincident technique employs a bidirectional microphone (with a Figure of 8 polar pattern) facing sideways and a cardioid (generally a variety of cardioid, although Alan Blumlein described the usage of an omnidirectional transducer in his original patent) at an angle of 90° facing the sound source. One mike is physically inverted over the other, so they share the same distance. The left and right channels are produced through a simple matrix: Left = Mid + Side, Right = Mid − Side (the polarity-reversed side-signal). This configuration produces a completely mono-compatible signal and, if the Mid and Side signals are recorded (rather than the matrixed Left and Right), the stereo width can be manipulated after the recording has taken place. This makes it especially useful for film-based projects."
 
 
 
 
 
With regards to generalizing that SDC are more effective at, or sensitive to, capturing high frequencies: That idea is based on the idea that the small diaphragms have a different range of resonance than large diaphragms and this is, more or less, true when you compare like to like examples of capsules that are made to a standard of very high quality construction. When you compare a high quality capsule to a discount commodity grade capsule you may find that any generalizations used to form an assumption will be a distraction to understanding the actual circumstance you have at hand. 
 
Trivia: SDCs do often times have a much more natural sounding off axis response. Indeed my Schopes MK41 cardiods don't seem all that special compared to many other choices until you listen deeply and start to notice that sound sourced from off axis is noticeably reduced in level while sounding very familiar and "natural". It's not something you think about much until you go back to a mic that doesn't do that as well and then you may realize that the character of off axis response has a substantial impact on one's impression of the overall sound. It is still not a characteristic that should be generalized as SDC vs LDC but it does seem to be the case that no top of the line LDC does it as well as some top of the line SDCs.
 
 
all the best,
mike
 
 
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silvercn what mics do you have available or were thinking of using. None of the above info is relevent (interesting though) unless you have the mics available. Might be good to know what you actually are intending to use.

Jeff - thanks: I have these: Figure 8: MXL R144; LD-ADK S-51 and Berhinger C-1; and SD-MXL-991. Certainly not high end, but these are the ones I am going to be using. 
 
I have also learned (unless not doing it right) that if I try to send each of the side channels to a reverb bus, they cancel each other out when they arrive there. 

hate to dissent... but one needs to use whatever one has on hand to learn, experiment, discover, etc. While it would certainly be nice to have a locker with lots of high end microphones of every variety, most of us don't have that luxury. So experiment. Do things wrong.
 
Speaking of which, you've already run into one lesson - the side channels are of opposite polarity (contrary to popular belief, they are not technically "out of phase"<G>) - so when you sum them together you will get quite a bit of cancellation. It won't be perfect of course, but it will be quite impressive... just not in a good way.