In Java, you can use the method File.lenght() to find out the size of a file in bytes.

The function  File.lenght()  returns the length in bytes of the file. The return value is undefined if the specified path is a directory.

After getting the file size in bytes, you can convert it as needed to megabytes, gigabytes; etc. But how do you do it? It's easy! A byte is a grouping of 8 bits and represents 256 possible values. So to convert a byte to bits, you just have to multiply by 8. The other units (kilo, mega, giga, tera) are placed in a hierarchy with the unit (n+1) above. is equal to 1024 unit (n) as shown here:

• one kilobyte (kilobyte) = 1024 bytes
• one megabyte (megabyte) = 1024 kilobytes
• one gigabyte (gigabyte) = 1024 megabytes
• one terabyte (terabyte) = 1024 gigabyte

This Java program creates an instance of  File with the path of a video file as a parameter and displays its size in bits, bytes, kilobytes, megabytes, gigabytes, and terabytes:

import java.io.File; 

public class main 
{
 public static void main(String[] args)
 { 
 File file =new File("C:\\test.avi"); 
 
 if(file.exists()){
 
 double bytes = file.length(); 
 double bits = bytes * 8; 
 double kilobytes = bytes / 1024; 
 double megabytes = kilobytes / 1024; 
 double gigabytes = megabytes / 1024; 
 double terabytes = gigabytes / 1024; 
 
 System.out.println("bits: " + bits + "bits"); 
 System.out.println("bytes: " + bytes+ " bytes"); 
 System.out.println("kilobyte: " + kilobytes+ "KB"); 
 System.out.println("megabyte: " + megabytes + "MB"); 
 System.out.println("gigabyte: " + gigabytes + "GB"); 
 System.out.println("terabyte: " + terabytes + "To"); 
 }else{
 System.out.println("Non-existent file"); 
 }
 
 }
}

The execution of this code gives:

bits: 4.624187392E9 bits
bytes: 5.78023424E8 bytes
kilobyte: 564476.0KB
megabyte: 551.24609375MB
gigabyte:  0.5383262634277344GB
terabyte: 5.257092416286469E-4TB

References:
Stackoverflow: How to get file size in java?
Wikipedia: Byte