WPF Login window appears smaller when application starts

  dialog, size, windows, wpf, xaml

I have a WPF application with a login window that appears before the Shell appears. In the application i have also added Logout functionality. My problem is that when the application first runs, the login window is much smaller than in design view, but it still reports that it has the dimensions set in design view. (I just set the dimensions in design view to test what was happening, otherwise i didn’t hard-code any dimensions)
Then, when i logout, the login window appears again, and this time it has the expected/desired size.
Now, it’s not only the window size that is different when the login view first appears, but the font is also scaled down. Here’s a view of the two login windows with different sizes:
enter image description here

And just for context, the login "window" is actually a user control shown as a dialog. The application is a WPF PRISM application.

Source: Windows Questions