Procopius said is best:
So the church has become a spectacle of marvelous beauty, overwhelming to those who see it, but to those who know it by hearsay altogether incredible. For it soars to a height to match the sky, and as if surging up from amongst the other buildings it stands on high and looks down upon the remainder of the city, adorning it, because it is a part of it, but glorying in its own beauty, because, though a part of the city and dominating it, it at the same time towers above it to such a height that the whole city is viewed from there as from a watch-tower. Both its breadth and its length have been so carefully proportioned, that it may not improperly be said to be exceedingly long and at the same time unusually broad. And it exults in an indescribable beauty. For it proudly reveals its mass and the harmony of its proportions, having neither any excess nor deficiency, since it is both more pretentious than the buildings to which we are accustomed, and considerably more noble than those which are merely huge, and it abounds exceedingly in sunlight and in the reflection of the sun’s rays from the marble. Indeed one might say that its interior is not illuminated from without by the sun, but that the radiance comes into being within it, such an abundance of light bathes this shrine.