Viral
0402 v 0603
that is not a simple topic.
And not really a topic for this forum- best on another forum to avoid polluting this forum
-so I will answer quick
- more board density. lower inductance useful for some applications. some parts are not available in 0402 only 0603 etc
0402 sized parts come on 8mm tape and usually 2mm pitch. . 0603 and larger and common parts SOT-23 etc come on 8mm tape and 4mm pitch.
I would suggest, sticking with 0603 until you understand the difference yourself. There is alot to it. if you are buying 8mm tape feeders, and you do not know how many 0402 compared to 0603 you might use, I would suggest perhaps order 10% (8mm x 2mm), and 90% (8mm x 4mm pitch) - BUT ! as I said , if you need to ask this question, my guess is your experience is not sufficient to make use of them.
Anyway, this is not the place for this sort of discussion- you will find I think plenty on answers on this blog :
"Projects, Designs, and Technical Stuff"
or
"General PCB/EDA/CAD Discussions"
good luck :-)