Hi

do you mean confinement?

If this is the case, you can predict the effect by solving Schrodinger equation 

you can also see it experimentally: shift of the emission bands to the high energy side compared to the bulk material band gap (photoluminescence )

Hope this is useful for you

Bests

Thank you so much dear Bouraoui,

Totally i am confused about size effect and i could not find any general equation for size effect. i would like to know that there is any general equation for size effect which could be applied for all materials? or each material has own theory? and own way?

You are welcome 

what do you have as a structure?

is it a layer, nano-crystals or 1D ?

To have an accurate help you should give more specifications of the investigated structures 

it is silicon nanowire

im really appreciated for your help

theoretically the diameter of the nanowire should be below the debroglie wavelength of silicon  to observe confinement effect