Adding window-size

I haven't tested Chrome; it's taken from symfony#428 (which can be closed now IMO)
Firefox works for me; it's taken from https://stackoverflow.com/a/64374604/1668200

README.md

@@ -388,6 +388,27 @@ $client = self::createPantherClient([
 * `PANTHER_FIREFOX_ARGUMENTS`: to customize Firefox arguments. You need to set `PANTHER_NO_HEADLESS` to fully customize.
 * `PANTHER_FIREFOX_BINARY`: to use another `firefox` binary
 
+### Changing the Size of the Brwoser Window
+
+This also controls the size of the screenshots.
+
+Chrome:
+
+```php
+$client = Client::createChromeClient(null, ['--window-size=1500,4000']);
+```
+or set it via `.env`:
+```
+PANTHER_CHROME_ARGUMENTS='--window-size=1500,4000'
+```
+Firefox:
+```php
+use Facebook\WebDriver\WebDriverDimension;
+$client = Client::createFirefoxClient();
+$size = new WebDriverDimension(1500, 4000);
+$client->manage()->window()->setSize($size);
+```
+
 ### Accessing To Hidden Text
 
 According to the spec, WebDriver implementations return only the **displayed** text by default.