Get Image File Size From Base64 String

Multiply the length of the data by 3/4, since encoding turns 6 bytes into 8. If the result is within a few bytes of 4MB then you'll need to count the number of = at the end.

I'm using this:

def size(b64string):
    return (len(b64string) * 3) / 4 - b64string.count('=', -2)

We remove the length of the padding, which is either zero, one, or two characters =, as explained here.

Probably not optimal. I don't know how efficient str.count(char) is. On the other hand, it is only performed on a string of length 2.