Before getting into file sizes, let's get familiar with the bits and bytes terminology.
These days, we don't measure kilobytes. This page is at least 5 kilobytes not including the pictures. You will be familiar with megabytes because cameras use this measure to show how big the pictures it will take. A typical camera today will boast a 10 megapixel resolution while some professional models will be over 20MB's.
The term gigabyte usually shortened to Gigs, will be used in measuring the memory in your computer or the size of a hard drive. Nowadays, hard drives are often 1000 gigabytes so they use the Terabyte. You will see drives of 1.5Tb's. This means 1500 gigabytes. I personally never have used the Petabyte term but I imagine it will be used in the future as files grow.
Example of typical sizes:
A one page letter in a word processor: 10 Kb's
A one or 2 page spreadsheet: 40 Kb's
A 350 page novel (e-book in pdf format): 1 Mb's
A picture using a average camera (jpg): 2 to 3 Mb's
A 3 minute song (MP3): 3 Mb's
A 1 minute High Definition Video (mov): 200 Mg's
A 1 minute TV quality Video: 40 Mg's