Use os.path.getsize(path) which will

Return the size, in bytes, of path. Raise OSError if the file does not exist or is inaccessible.

import os
os.path.getsize('C:\\Python27\\Lib\\genericpath.py')

Or use os.stat(path).st_size

import os
os.stat('C:\\Python27\\Lib\\genericpath.py').st_size 

Or use Path(path).stat().st_size (Python 3.4+)

from pathlib import Path
Path('C:\\Python27\\Lib\\genericpath.py').stat().st_size

It's not clear from your question whether you want to the compressed or uncompressed size of the file, but in the former case, it's easy with the os.path.getsize function from the os module

>>> import os
>>> os.path.getsize('flickrapi-1.2.tar.gz')
35382L

To get the answer in megabytes you can shift the answer right by 20, e.g.

os.path.getsize('large.tar.gz') >> 20

Although that operation will be done in integers - if you want to preserve fractions of a megabyte, divide by (1024*1024.0) instead. (Note the .0 so that the divisor will be a float.)

Update: In the comments below, Johnsyweb points out a useful recipe for more generally producing human readable representations of file sizes.

Use os.path.getsize:

>>> import os
>>> os.path.getsize("/path/to/file.mp3")
2071611

The output is in bytes.

Try referring to this Stack Overflow post. They are looking at zip files, but it should work fine for your raster file.

If you are using a type of raster that is made up of multiple files, then I would suggest getting the file size of each and summing it.

The code would be similar to:

import os
os.path.getsize(#your file here)

As the post says, you can shift it to MB like this:

os.path.setsize(#your file here) >> 20

Credit definitely goes to Mark Longair and his answer on the referenced form post.