The Paint tool 'F2' has both round and square. The standard paint brush is round, and the diameter is controlled by the Radius '+/-' knob. To fill a square, select the Select area 'B' tool, then select an area, then press the Fill area 'F' button and it will fill the square.
You can also play with the Direction '[/]' and Scale Ctrl '[/]'. These two are important to break up the patterns of some of the texture that tend to look repeating, or thing like cliff textures that need to be in a specific direction, like horisontal, not vertical. These are generally used with the brush, not fill.
You can also change your baseboards from the standard 10m grid, to a 5m grid, that way, when painting, you will have a higher level of detail.