[Jellby]

If you don't want to modify the text and you want all letters (uppercase or lowercase) the same size, you could use text-transform, but that's not a supported property in the ePub spec, so it's very unlikely to work in real-world readers:

span.smallcap {
  text-transform: lowercase;
  font-variant: small-caps;
  font-weight: bold;
}

or

span.smallcap {
  text-transform: uppercase;
  font-size: 80%;
  font-weight: bold;
}

When real small caps are available and supported (very rare), the first option is preferred, as a smallcapped "a" is not the same as a smaller "A" (the stroke thickness should not be scaled down, for example). But most readers just ignore font-variant, so the second option, although, sub-optimal, is more likely to work.

In any case, as I said, text-transform is not required in the spec, so don't rely on it. If you really need uppercase or lowercase text, just modify the text. But I guess in this case it's only a secondary aesthetic choice, and not much is lost if a given reader ignores the text-transform.