Hello Newbie1 and welcome to the ArtRage forums. I hope you are enjoying ArtRage so far. To answer your question, the option you want to use to change the scale of the tools marking area is found in the Tool panel, located at the lower left of the ArtRage screen (Please refer to my attached image, for more details). For future knowledge, please keep in mind that zooming the canvas only allows you to get a closer look at your painting, so the tools scale remains the same in relation. The Pressure option you tried, found on the Settings panel, will react differently between tools. For some, such as the eraser, it will allow the user to change how much color the tool erases from an area in a single pass. Visually, it may appear to become smaller or larger, but that's probably because of the softness, which fades the edges of the tool. Since the soft edge will remove less and less color as the pressure is lessened, the scale of the tool may appear to change.
Nothing is easy to the unwilling.