Dear Ali,

Needle to collector distance is one of the parameters that can affect the electrospinning process since it determines the electric field ( in kV/cm ) for the fibres to form and the distance/time there is for the solvent to evaporate. However, from my experience, it is not one of the main variables in the process and its effect is not very significant on fibre morphology or formation. Other variables such as polymer concentration, the choice of solvent and the applied voltage will have a much more strong effect on the formation and morphology of fibres. You can read the attached review to begin with. Hope this helps!

http://www.hindawi.com/journals/jnm/2013/789289/

You may get the picture from below.

Ramakrishna, S. (2005). An introduction to electrospinning and nanofibers, World Scientific Pub Co Inc.

The distance from the needle tip and the collector is also one of the variables. Higher the distance the thinner should be fiber dia. 

please check this paper. http://www.sciencedirect.com/science/article/pii/S0011916414005190