If you're using Figma for prototyping, there are two ways to make sure your prototype scales correctly to your participant’s device or browser window.
Building your prototype
In prototyping mode, you can select a device or set a Custom Size to fit a window. Alternately, you can select Presentation to fill a window:

Viewing your prototype
While viewing the prototype, you have 3 scaling options:
100% (potentially cutting off part of your design)
Fit (scale down to fit any screen)
Fill (scale down or up to fit any screen)
You can find those controls by hovering in the upper-right-hand of the prototype screen:

Recommended: test internally before sharing with your participants
We strongly recommend testing the settings yourself or with a colleague, to make sure your prototype works as expected with our Participate apps. For plans that are session-limited (Trial, Freelance, Team, Insights Hub), you can use our Preview Links to test without using up any of your live/full sessions.