Besides Selective Channeling, there is no way to change the area of the channel aura(Although the Holy Vindicator can do it). One can't pull the aura in to help avoid enemy targets or widen it out to catch those who have spread out. Perhaps we could think of alternate rules to allow more flexibility?
1.A cleric can extend their channel aura by 5-10 feet by sacrificing 1d6 of healing/harm.
2.By reducing their aura 10 feet, a cleric can increase healing/harm by 1d6.