browser javascript python selenium selenium-webdriver

How to set browser viewport size

I’m trying to create a cross-browser Python-Selenium test script. So I need all results to be same no matter which webdriver (Chrome or IE) I use.
I can set browser window size as follow:

driver.set_window_size(1920, 1080)

But following code will return different values for Chrome and IE:

element = driver.find_element_by_xpath('some_xpath')

as viewport area (where web-content displayed) sizes are different (Chrome– 1910×998, IE– 1904×965) despite of same window size. To get these values I used

driver.execute_script('return document.documentElement.clientHeight')
driver.execute_script('return document.documentElement.clientWidth')

so I tried

driver.execute_script('document.documentElement.clientHeight = "990px";')
driver.execute_script('document.documentElement.clientWeight = "1900px";')

but with no luck

So the question is how to set browser viewport size in selenium?