onPushButtonClick code redirects to another Widget/Class(mainWindow.cpp)

I am wondering how I can redirect my code into mainWindow.cpp where my main program will be located after the user clicks on the "login" button(onPushButtonClick). Currently when the user pushes the button it opens the second widget but it stays at the smaller size equivalent to the login window, so how can I redirect the code in mainWindow UI so after they click "login" it opens a new window at the correct size as my MainWindow.ui.

Picture of code