Q:

How to get image size (bytes) using PIL

I found out how to use PIL to get the image dimensions, but not the file size in bytes. I need to know the file size to decide if the file is too big to be uploaded to the database.

A:

Try:
import os
print os.stat('somefile.ext').st_size

A:

If you already have the image on the filesystem:
import os
os.path.getsize('path_to_file.jpg')`

If, however, you want to get the saved size of an image that is in memory and has not been saved to the filesystem:
from io import BytesIO
img_file = BytesIO()
image.save(img_file, 'png')
image_file_size = img_file.tell()

This method will avoid multiple reads of the image data as with StringIO. Note, however, that it will use more RAM. Everything is a tradeoff. :-)
Edit: I just saw this comment from the OP:

finally, the problem is from the beginnig, if someone will upload a
  picture that have 1 giga (forged one) he'll kill the server before PIL
  will do its stuff, so i must block the request before it finishs!

This is a very different question, and is probably best accomplished at the web server. For nginx, you can add this to your configuration:
http {
    #...
        client_max_body_size 100m; # or whatever size you want as your limit
    #...
}

A:

I think this is the true measure and the fastest one of the size of the image in bytes in memory:
print("img size in memory in bytes: ", sys.getsizeof(img.tobytes()))

Then, the size of the file on disk depends on the format of the file:
    from io import BytesIO
    img_file = BytesIO()
    img.save(img_file, 'png')
    img_file_size_png = img_file.tell()
    img_file = BytesIO()
    img.save(img_file, 'jpeg')
    img_file_size_jpeg = img_file.tell()
    print("img_file_size png: ", img_file_size_png)
    print("img_file_size jpeg: ", img_file_size_jpeg)

Possible output for 32 x 32 x 3 images from CIFAR10 dataset:
img size in memory in bytes:  3105    
img_file_size png:  2488
img_file_size jpeg:  983

