Web app integration popup change popup window size

For the webapp integration popup type we can have a popup window with our rendered html/js, and I can run window.resize() function to resize the window. But unfortunately the popup first shows up as default size before resizing to the size I want-- creating a jarring experience for the user. 

I notice some integrations: Notably slack web app integration whose popup immediately appears as the desired size. 

Was wondering if there was a setting somewhere where I can specify the window size so it is the proper size from the get-go?