Hello Shaikss
If you are into the field of antenna's, First thing that dictates the requirement is your substrate and dielectric constant. For instance you want to fabricate an antenna for a specific purpose,you will have the physical and electrical constraints that go in for choosing the substrate. so what is your antenna being used for in which condition? that answers your first question.
How to determine the dimensions, there are equations governing the values of L,W and so on.Refer a book an antenna theory,my suggestions Balanis or pozar.
How do you draw and co ordinate your patch,ground plane and feed. first feed using the equations determine where is your 50 ohm point or the point of resonance . thats your feed point and then patch dimensions are calculated from the step 2 from the book. ground plane, make sure the ground plane is large enough compared to the patch and check the infinite ground plane when you set up the boundary condition for the patch,if required.
There are so many of these tutorials which tear down the complications to such a basic level.
Hope that helps.
Rotmanlens.