Importance of brush strokes in your painting. By Don Finkeldei: This article explains the best use of brush size, Stroke length, direction and detail for each stage of a painting.

I've found that most beginning artists (I did this too) make all strokes either vertical or horizontal without variety of stroke size, pressure, color intensity, value, or direction ... and with little regard about the object relating to the other value planes.  There is no need to focus on detail right from the start.

A large brush should be used to establish the modeling of each object within each respective value plane.  The broad value planes in a landscape are sky, horizontal, slanted and uprights (defined and explained explicitly by John Carlson).  There are also objects within each value plane that need to be modeled into the 3 areas (light side, transition area, and shadow).  Each object within it's value plane has relationships to objects and lighting conditions in each of the other value planes.  See this article.

I consider a painting to be a 3 stage process.

1. Initial block in stage.  Large areas of similar value, hue, and color saturation for each object within a value plane.  There are shadow areas, transition areas and full lighting areas for each object in each plane (sky, ground, vertical and slanted planes).  There are relationships of values between each value plane. See light and value planes.  Establish all of these larger modeling areas with one or a few bold strokes with the largest brush you can use. Use different angles and pressure of brush strokes. Break up objects with unequal measures (space differently, no repetition ).  Don't pick at them or daub at them without thinking.  The areas can be adjusted with detail at a later stage.  Keep from designing lolly pop trees.  Trees have a huge variety of shapes.
   
   
2. Modeling Stage.  This is where the detail starts to emerge within each area of an object in each value plane.  A tree, a building, a line of trees, a river, rocks, etc.  A smaller brush at this stage is good... but not too small.  Change value very slightly and/or change the hue keeping the value the same (Most great paintings are created by changing hue but keeping the value the same). That's the detail within each object... and related to each value plane.
   
   
3. Finishing Stage.  This is when you add the accents.  You can use a small brush at this point... or even the edge of a palette knife.  Accents can be brush strokes that are either higher value or lower value than than the general modeling.  A bright aspen tree trunk, a few leaves on the light side of a tree, the light glinting off a roof of a building.  It can also mean a few brush strokes of very dark value areas at the base of trees or brushes or other objects.

Remember that you also have to control edges with your brush.  I usually do this after boldly adding a stroke.  Generally use a few hard edges where value contrast is greatest.  Normally in the foreground and/or in the focal point area.