How to make your quilt block any size you like.

The secret to making your quilt block any size you like is drafting it to scale on graph paper or an appropriate computer drawing program with a grid.
Usually this will be the standard square paper unless you plan to do triangles or hexagons, but graph paper to print out to draw these is available on the internet too.
Once you have your square paper and have drawn your pattern you can then decide how big you want the finished size of your block to be.

Lets use a simple example of a shoo fly block

This is made from a 3x3 grid.  If I want my finished block to be 12 inches then each of the squares would be 4 inches - finished size.  That is to say that the drafted drawing does not have the seam allowances shown.  For each square on the gird we need to add 1/4 inch seam allowance on all sides.  For squares and rectangles that means that we calculate the length and height of each shape and add 1/2 inch to both measurements.  e.g in the above example we want 4 inch finished squares so we would cut 4 1/2 inch squares.  Triangles are a little trickier as they have to be cut larger to cope with the fact that the points reach out past the square.  To work out the size of the half square triangle in the above example take the size of the square you would need (4 1/2 inches) and add another 3/8 of an inch - regardless of the size of the half square triangle you need; so we would cut 4 7/8  inch squares and cut these in half diagonally to make the corner half square triangles.  If you are not that great with half square triangles then round to the nearest inch and square up the half square triangle units to the measurement desired once you have pieced them.
For more complex shapes it is generally a good idea to make a template of the shape to the size that you need and then add the 1/4 inch seam allowance on all sides, use this bigger template as the pattern piece for your measurements.