The recent upgrades I was involved with also used 22 ga/25 pair direct burial gel filled rodent resistant telco grade. I can't stress enough how important it is to not cheap out on the buried cable product. If you must skimp on budget, do it on the above-ground portions of the work like boxes and terminations. At least those are accessible and can be upgraded later. The buried cable is there for decades; the misery of unreliable signals or insufficient capacity will last far longer than the fleeting satisfaction of a 'bargain'.
25 pair is less than double the price of 12 pair, which in turn is less than double the price of 6 pair. When in doubt, go with the larger size. The additional cost is just pennies a year when applied over the lifetime of a well-installed system.