[Gavino]

For technical reasons, the final width must be greater than the number of 'taps' on the resize filter used, so a width of 1 is not possible with any of the resizers (except of course PointResize).

But even if it was permitted, it wouldn't give you a plain average anyway - it would be a weighted average, with different weights for each pixel depending on its distance from the centre.

One way to get what you want is a horizontal box blur with a box whose width is equal to the image width, and then crop out the centre column as your final result. However this only works exactly if your initial image width is an odd number.