One might be tempted to think that the horizontal spacing and vertical proportions of the altar candlesticks and cross are not particularly important to an altar and church so long as the traditional six (or "big six" as they are often colloquially referred) are used, but the particular arrangement of these elements can significantly influence how an altar is visually perceived and, what's more, it can set the altar into a better overall proportions to the rest of the church -- and especially in relation to a reredos or ciborium magnum if present.
Now by "horizontal spacing" I refer to the manner in which the candlesticks and altar cross are lined up across the mensa or gradine of the altar. By "vertical proportions" I refer primarily to the combined height of the candlesticks and the candles within them -- though I would add that the substantiality (or visual weight if you prefer) of the candlesticks themselves are a critical factor in addition.
To demonstrate the point, I thought it might be of interest to consider two famous altars, those of the papal basilicas of Santa Maria Maggiore and St. Peter's in Rome. These examples are particularly good for this sort of consideration because both have ciboria, both are large basilica churches, and both have altars and altar arrangements that are reasonably traditional to this day. In this regard, it makes for a good "apples to apples" comparison.
To begin with then, let's take a look at the high altar of Santa Maria Maggiore, comparing a 19th century photograph of that altar (left) with its contemporary counterpart (right):
The 19th century arrangement shows candlesticks and cross that reach up far higher than their contemporary counterparts (the altar cross also having much more visual weight let it be noted). What's more, while the 19th century arrangement has the candlesticks and cross evenly spaced across the mensa, the contemporary arrangement sees uneven spacing between the candlesticks and cross.
While both arrangements have a dignity about them, it seems to me that the 19th century arrangement not only sets the altar into better overall proportions to the ciborium, it also has the effect of giving the altar greater visual prominence and centrality generally. Beyond this, I believe the even spacing of the candlesticks and cross lend the whole arrangement a greater balance, symmetry, harmony and ultimately give the altar a greater overall simplicity.
Here is another variation on this same theme and, while not quite as tall as the 19th century version above, it has the same good effect. (This photo also shows quite well the benefits of the even horizontal spacing the candlesticks and cross on the altar):
The black and white photograph is difficult to see, so here are a couple of closer looks:
In this instance, both arrangements utilize even horizontal spacing for the cross and candles. The real difference here rests in their respective vertical proportions -- as well as in the visual 'weight' of the candlesticks themselves.
Rather than repeat myself, I would simply note that I would make the same assessment here as I did for the altar of Santa Maria Maggiore: namely, that the taller candlesticks and candles better accentuate the altar itself and set it into a better overall scale and proportion to the great baldacchino of St. Peter's.
The principles I would suggest we might find here are as follows:
First, even horizontal spacing of candlesticks and cross is preferable to uneven "clustering" of the same. It is orderly, balanced and has greater visual simplicity.
Second, when dealing with ciboria, tall reredos, or basilica and cathedral sized churches generally, taller and weightier candlesticks better ensure that the altar is set into proportional scale with the whole and also ensure its centrality and visibility.
Elements such as these can be all too easy to neglect, but I think you will agree that they can have a decided impact.
For more thoughts around this subject, also see Considerations of Candles and Their Proportions in Relation to the Altar and Lessons in Liturgical Arrangement Found in the Pantheon.
