You will need to do a lot of explaining! File size - number of records? number of bytes? nmumber of blocks? number of tracks? number of cylinders? number of tapes? File can vary in size, record-wise, from zero to millions of records. same sort of thing for the othe other criteria.
Of course, you could always follow the forum rules and search:
a previous topic
And after 3 months in a technical role you should know how to do this - not wait for 6 years. This is a beginner's question. why not join the this sites sister forum for beginners?
LRECL x Number of records - is a rough rule of thumb for total bytes.
Another way is to view the file in 3.4 and get the number of tracks used, and assuming you're on 3390 DASD then multiply the number of tracks by 56664 (no of bytes per track). It's not 100% accurate but probably close enough. At least this way it shows what is actually used regardless of the BLKSIZE.
BLKSIZE can have a big effect on file size. If the fle is allocated using System Determined Blksize (SDB) by not specifying BLKSIZE or coding BLKSIZE=0 then it will get the most efficient half track BLKSIZE. But if someone has done something silly like creating a file with RECFM=FB,LRECL=80,BLKSIZE=80 then the file will be multiple time bigger than it needs to be due to most of it being 32 byte interblock gaps.