By clicking onto the third button at the screen's bottom edge you can open the Toolsize Menu where you can choose the size of the area you affect with the tool you select from the Tool Menu. For example, selecting a particular terrain type and switching to Toolsize 5 will let one change the terrain type of the triangle that the mouse cursor is pointing at and all triangles that can be reached from it by crossing at most 4 edges. Most tools operate on nodes rather than triangles. For those tools, Toolsize 5 will affect the node at the mouse cursor position and all nodes that can be reached from it by following at most 4 edges. In both cases, symbols will be shown on the map on all triangles/nodes that are within the selected range.
A much easier way to change the Toolsize is by using the hotkeys, which are 1 through 0. The number corresponds to the Toolsize you choose, for example pressing 6 would select Toolsize 6. The 0 means Toolsize 10.