Hi Rediska ,
First , the question you have asked is not really a question ; why are you concerned with file size ?
File size , per se , has nothing to do with file open / file save / calculation times , which is what you should be concerned with.
Secondly , certain basic calculations , or calculations which have built-in Excel functions to do the job ( e.g. Engineering , Statistical , Financial , Trigonometry ) can and should make use of these , unless you find that they are either not precise enough , or fail to give the correct results in all cases.
VBA should be resorted to if you need to go above and beyond what native Excel functions can do.
Narayan