The ratio of 2:1 was the standard for ensigns of the United Kingdom. As the CSA was seeking recognition of the UK, I am not surprised that they modeled their flags on those of Britain. (British ensign proportions standardized on 2:1 early in Victoria's reign, though the exact date is not certain.) Since it seems that at least some of the South's flags were made in Engiand, they may have received 2:1 flags whether they asked for that proportion or not.

Even today, the flags of many Commonwealth nations follow the 2:1 proportions, including Canada, Australia, and New Zealand. The flag of the USA (initially a group of former British colonies) has official proportions of 19:10, which is darn close to twice as long as it is wide. I always suspected this long aspect ratio allowed the fly end to be repaired/rehemmed numerous times before the flag became unserviceable.

Nick