When I arrange vertex in a shape in front perspective view and when I move them forward from side view the shape looks bigger as it comes forward in front perspective view and when every time I move it forward I need to shrink it to make it look same the same size as it looks before, this takes a lot of time for modeling. Is there any way to shrink the model as it comes forward so that it looks the same size as it looks before?
Perspective, while it is the way we see things in the real world, is an illusion.
These train tracks look like they are converging, but obviously they can't be.
Orthographic projection is a way of seeing 3D shapes in 2D at their actual sizes, without perspective distortion.
Here is a comparison of orthographic versus perspective, with the same camera angle.
The orthographic version doesn't look "right", but the tracks appear parallel, which is how they actually are.
By moving the camera backwards and increasing the focal length, you can kind of get an ortho-like look.
Assuming you are modeling from an orthographic or nearly orthographic reference image (obviously it is impossible to get a true orthographic projection from a camera, but photographs can be "close enough" if taken with a long focal length), modeling in ortho is the best way to go. It does take a little getting used to though..
Another way of considering it.
Ortho view is when you want to look directly at the object from the TOP, BOTTOM, or the SIDES (front, rear, left and right). A cube would then look the same from each of these six views. So would a sphere (kinda). Especially in wireframe. But in perspective mode the bottom of the cube from the TOP would appear smaller than the actual top of the cube object.
So why use ortho view? This is especially helpful when making sure vectors are lined up on top of each other. Consider making a skyscraper. If you are looking down in perspective view (in wireframe), the walls are going to overlap as the object appears to get smaller near the bottom (distance). Switching into ortho view makes things appear as they would to an architect. It looks like a cross-section of the building.
When you rotate the view out of the six positions, it will still be in ortho, but the view is actually called isometric. It may look 3-d but it doesn't show depth over distance like the example above with the train tracks illustrates.
Your numeric keypad controls all of these features. 5 Toggles from pers to ortho. The other keys are snapped rotations with 1, 3, 7 being the most common direct views.