In Flex, you can set minimum and maximum widths or heights for the canvas, containers, and individual assets. When applied, they act as responsive caps when a viewer's screen size changes.
For example, applying a minimum width and height of 200 pixels to a square-shaped image with relative sizing means the image will continue to scale down as the canvas size shrinks until it reaches 200 pixels. At that point, the image won't reduce in size any further.
Or you have a text box with a relative width and set a max width of 800 pixels. As the canvas width expands, the text box will grow with it to keep its relative width until it reaches 800 pixels. At that point, the text box's width won't scale any further, even if the canvas continues to expand.
How to Add a Min and Max Size
The process for applying a min and/or max size is the same for width and height:
Select the target object on the canvas
Navigate to the Size section of the Inspector Panel while in Design Mode
Select the + icon (Add min/max width/height)
Select the desired cap from the dropdown menu
Min width
Max width
Min height
Max height
Select the sizing type in the corresponding field in the Size section of the Inspector Panel
Fixed or Relative
Input the desired min/max size
You can only set min/max height to the canvas and sections, not width. The process to add a min/max height is the same as outlined above. A minimum height of 900 pixels is automatically applied to the canvas when you add responsive variants; however, it can be adjusted or removed.