So if surface area is really big then friction will depend on surface area, i think that its approximately independent of surface area because when surface area increases then real area of contact at microscopic level also increases and as real area of contact increases then pressure on asperities decreases and the deformation of asperities takes place slowly and thus real area of contact increases with slow rate and thus friction increases with slow rate then compare to the friction between smaller area of contact between the surface.
And if area of contact is huge then it will increase area of contact with big amount and friction will increase with high rate
This is just a theorotical approach i understood till now, I don't have proof for this, but till now this satisfies me

The story goes that a large surface is just an assembly of small surfaces all with the same friction characteristics and the normal load on the large surface just gets shared out as small loads on the small surfaces .

How would you solve this question:
A body slipping on a rough horizontal plane moves with a deceleration of 4m/s^2. What is the coefficient of kinetic friction between the block and plane ?
Please note that this is not a HW question
I am asking this, because i think that in this case you will take -4 = -f(static friction)/m
And thus 4= f(static friction)/m
f(static friction) = μN = μmg
Therefore μ=4/g
In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating
Then how will you explain it at molecular level.

You said opposite things. First you say it is decelerating (slowing down; eventually to stop). Then you say it accelerating (speeding up). You are confusing yourself.

Kinetic friction resists motion, that is correct.
The velocity is positive so F_{kinetic friction} is negative and so the acceleration is negative (=deceleration).

PS: F_{kinetic friction} is not a constant force. As soon as the velocity reaches zero F_{kinetic friction} becomes zero and is replaced by static friction. So F_{kinetic friction} cannot make the block accelerate in the opposite direction.

If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case

I could solve problem containing two or three surfaces but solving questions containing only two surfaces sometimes makes me confuse but now i am cleared
Thanks @jbriggs444 and @anorlunda