Measure website home page total network size in bytes
You're using the wrong shebang. According to this StackOverflow answer:


  Correct usage for Python 3 scripts is:

#!/usr/bin/env python3

  
  This defaults to version 3.latest. For Python 2.7.latest use python2 in place of python3.


Don't use #!/usr/bin/python. From the Tutor mailing list:


  Consider the possiblities that in a different machine, python may be installed at /usr/bin/python or /bin/python in those cases, the above #! will fail.
  For those cases, we get to call the env executable with argument which will determine the arguments path by searching in the $PATH and use it correctly.

I'd move command line argument handling to the bottom, possibly in a main(). The rest of the code could be separated into function 'units'.
Certain variables and literals could be replaced by constants. From what I can tell:


logging_prefs could be a constant;
Waiting times could be constants;
Exit codes could be constants.

Catching Exception is bad practice.
These parentheses:

d = (datetime.today()).strftime("%d-%m-%y-%H-%M")


are superfluous.
This cast:

filename = 'results_{}.txt'.format(str(d))


is pointless.
Don't mix single and double quotes.
Don't assign multiple variables on a single line.
The same driver will be instantiated, regardless of the 'datacentre' argument. Therefore, the if-statement can be left out altogether.
Error messages should be written to stderr, not stdout.
Imports should be placed at the top of a file, and rarely in a function.


Here's my take on it (untested):

#!/usr/bin/env python3

""" This script will simply check the download page size (bytes) of a Home page."""

from __future__ import print_function

import argparse
from datetime import datetime
import re
import sys

from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities

LOGGING_PREFS = {"performance": "INFO"}

WAITING_TIME = 30
TIMEOUT = 30

DATA_LENGTH_REGEX = r"encodedDataLength\":(.*?),"

EXIT_OK = 0
EXIT_WARNING = 1
EXIT_FAILURE = 2


def get_driver(local):
    if local:
        caps = DesiredCapabilities.CHROME.copy()
        caps["loggingPrefs"] = LOGGING_PREFS
        return webdriver.Chrome(desired_capabilities=caps)
    else:
        profile = webdriver.FirefoxProfile()
        profile.set_preference("plugin.state.flash", 0)
        profile.set_preference("webdriver_enable_native_events", False)
        profile.update_preferences()

        caps = DesiredCapabilities.FIREFOX.copy()
        caps["loggingPrefs"] = LOGGING_PREFS
        caps["acceptSslCerts"] = False

        return webdriver.Remote(
            command_executor="http://selenium/hub",
            desired_capabilities=caps,
            browser_profile=profile
        )


def get_homepage_size(driver, website):
    driver.implicitly_wait(WAITING_TIME)
    driver.set_page_load_timeout(TIMEOUT)
    url = "http://" + website + "/"
    driver.get(url)
    total_bytes = []

    try:
        for entry in driver.get_log("performance"):
            entry = str(entry)
            if "Network.dataReceived" in entry:
                r = re.search(DATA_LENGTH_REGEX, entry)
                total_bytes.append(int(r.group(1)))
    except Exception:
        # TODO Find a more specific exception.
        # What could fail?
        driver.close()
        print("Failed to get data size for {}".format(website), file=sys.stderr)
        sys.exit(EXIT_FAILURE)

    if total_bytes is not None:
        return round((float(sum(total_bytes) / 1000) / 1000), 2)


def write_results_to_file(website, size_in_mb):
    date = datetime.today().strftime("%d-%m-%y-%H-%M")
    filename = "results_{}.txt".format(date)
    with open(filename, "a") as f:
        f.write("{}, {}\n".format(website, size_in_mb))


def display_results(website, size_in_mb):
    if size_in_mb &lt; 2.0:
        print("OK. Total Network Data Received size for {}: {}MB".format(
            website, size_in_mb)
        )
        sys.exit(EXIT_OK)
    elif size_in_mb &gt;= 2.0 and size_in_mb &lt; 4.0:
        print("Warning. Total Network Data Received size for {}: {}MB".format(
            website, size_in_mb
        )
        sys.exit(EXIT_WARNING)
    elif size_in_mb &gt;= 4.0:
        print("CRITICAL. Total Network Data Received size for {}: {}MB".format(
            website, size_in_mb)
        )
        sys.exit(EXIT_WARNING)


def main():
    parser = argparse.ArgumentParser(description="This script will measure size of a home page.")
    parser.add_argument("--site", default="somewebsite.com", required=True)
    parser.add_argument("--local", action="store_true", default=False)
    parser.add_argument("--datacentre", choices=["dc1", "dc2"])
    args = parser.parse_args()

    driver = get_driver(args.local)
    size_in_mb = get_homepage_size(driver=driver, website=args.site)
    if args.local:
        write_results_to_file(args.site, size_in_mb=size_in_mb)
    display_results(args.site, size_in_mb=size_in_mb)

if __name__ == "__main__":
    main()