Schematic plot of the 1s electron density function ψ1s2.
The "surface" of the orbital represents points for which the electron density for that orbital is the same - an isosurface. By choosing different values of electron density, denoted by the bar moving up and down on the line plot or by the moving plane on the surface plot, then the size of the three-dimensional plot changes.
As the wave function is directionless, so is its square. All values are of necessity not negative since the square of any real number cannot be less than zero.