Kuika Boost makes use of responsive behavior annotations that exists in an uploaded Sketch file. In places where these annotations don't exist, elements on a page stretch in width and height to reflect changes in their containers (Sketch Layer or Artboard). This results in shapes being squeezed or stretched horizontally and vertically.
Easiest way to test how a screen behaves in KuikaBoost is using the landscape/portrait rotation button on the toolbar. If your screen design resolution is not square, this can quickly show what happens when both screen width and height changes.
There are several steps to making a design behave properly with changing screen size
1. Convert layout of key elements to "stack layout". This ensures their content determine their size. For example, for a page whose height is based on the content in it, convert it to Vertical Stack" so it extends as tall as needed for its content
2. Fix width/height of elements that have a fixed appearance. Most logos, icons, etc on a page need to have a fixed appearance. If container of a logo/icon does not have fixed size, then fix width and height of the logo/icon element. Another common action is fixing height of header and footer on a page.
3. Fix the margins of elements to ensure their alignment looks good when screen dimensions change. For example in a top menu, items on the top left of the screen can have fixed left margin and items on top right of screen have fixed right-margins. Another common example is a paragraph of text where fixing left and right margins of text makes the paragraph adjust its width while keeping the space around it as designed.