The Simulink Toolstrip in R2019b has changed the location of a few things, but the variable size array configuration is still available. The first thing to do is to set up your model to be used with ROS by selecting the Robot Operating System (ROS) app. This should configure all your hardware settings for you (though you can choose between ROS and ROS 2 depending on which you are using), so you can just hit OK. Then the Variable Size Message configuration option becomes available on the main Simulation tab in the ROS Toolbox subsection. There is also a new Robot tab that is useful in other workflows.
See the image below for a quick guide. Hope this helps!