Dear Srjani,

The first indication for nanoparticles come from compared energy gap for this film as a bulk and nano thin film and if value of energy gap increased compared with the bulk in approximately 1 eV , we can say the thin film tend toward nano, and this mean when the grain size reduce then the shift toward blue region obtained. and here many reasons can be give you in future. With best regards 

Dear Srjani,

Yes you can use the same formela to calculate the particles size because X-ray beam covere around 1x0.5 cm of the sample area.

Dear

you can measure grain size using  XRD, SEM and AFM 

you will never get same result since XRD is measure the grain size in Z- direction

while SEM and AFM  measure grain size in XY-direction