The exclamdown should be (almost) exactly the height of the exclam. So, if you are going to make a special exclam and question for "all small caps," you need small cap "downs" as well. You will need the regular sized ones for cap-smallcap settings.

As we discussed on the Design Forum recently, the question of how to work the OT features depends on the applications programs capabilities, and how you expect the font to be used. But I would think that for a titling font, you can expect any reasonable comp to take some pains getting things right, so the toggling of features is less important.

Sorry Charles,
I think I worded that wrong. I think the size should be the same. I was referring more to positioning. On the Microsoft standards site it say there are 2 ways.

"The inverted question and inverted exclamation have two possible alignments."

"X-height alignment : In most text fonts the inverted question and inverted exclamation tops align with the lowercase x-height. In a round design the lowercase x-height's overshoot. The bottom should not exceed the lowercase descender value. In this case many type designers use an uppercase alignment method or shorten and redesign the glyphs."

"Uppercase alignment : This is a simple 180 degree rotation with no vertical offset from the position of the question and exclamation. Used very commonly in display fonts and modern designs."

It doesn't mention Smallcaps, but I'm inclined towards using the Uppercase alignment throughout.

Why don't you insert two exclamdowns.
The default one should have the same height as the small caps, while the other one "exclamdown.case" (applied via case-feature) should be as tall as the UC.

Spanish-speaking friends (well, editors) of mine say that preceding a capital letter, the exclamdown and questiondown should align at what you & Microsoft are calling the "uppercase" position. Preceding a lowercase letter, the "x-height" position can be used.

As Christoph says, I would have two for a Cap/smallcap display font. There will be people who will use only the small caps for a heading, and people who use Cap/smallcap.

Good point!

Thanks to both of you.