Many like to equate the size of the voice with its ability to be heard over an orchestra. The assumption is that a large voice will be heard more easily than a smaller voice. Of course, that depends on what is meant by a “large” voice but, based on the comments I have read, most consider a large voice to be one that has a broad or extremely full tone and some on this list consider a voice that can yell in tune to be even more acceptable.
But consider the oboe and how easily it can be heard within a rich orchestral spectrum. Or the trumpet, or even the bassoon to say nothing of the solo violin. Each of these instruments have carrying quality because they have a sonic emphasis in the range to which the human ear is most acutely attuned. That range? Approximately 2800 to 3600 Hz. And any voice that has a sonic emphasis in that range will also be easily heard over even the largest of orchestras.
In other words, when we in the audience are unable to hear a singer over the orchestra, it means that the singer is not producing a sonic boost in the aforementioned range of tones. Keep in mind, that singers do to not sing pitches, that is, fundamental tones, in these ranges. A C7, for example is four lines above the treble staff and no singer is ever required to sing that high yet it is only at about 2093 Hz. The necessary 2800 to 3600 Hz emphasis is the singer resonating one of the overtones of their sung pitch. And all singers can be trained to do this, from the lowest bass to the highest coloratura.
For example, if Rene Fleming, a lyric soprano, produces a “Singer’s Formant” (2800-3600 Hz) she will be easily heard. Even a light lyric such as Nellie Melba could also be easily heard. Any singer not heard, simply needs to develop the technique that produces such resonance richness.
Now, all of the above does not mean that any soprano could sing any role. Roles are determined less by the ability to sing the required notes and more by the quality of vocal tone necessary for the dramatic demands of the role. Casting is, and always should be, based on the tonal demands of the role. The ability to be heard over an orchestra should simply be a given; it must be there for any singer.