INT 21,23 - Get File Size Using FCB

	AH = 23h
	DS:DX = pointer to an unopened FCB

	on return:
	AL = 00 if successful
	   = FF if file not found

	- determines the number of records in a file
	- FCB must be setup with drive id, complete filename and
	  extension plus record length before call
	- updates random record position in FCB located at DS:DX
	  with file record count