Yes, this can be done but it won’t auto-fit, it’ll only work perfectly if your mock-ups use the same pixel width as the phone you are viewing it on. So if you design for iPhone 14 but you’re using the Figma all on iPhone X it won’t look perfect.

How to toggle presentation mode to "just the screen without mobile phone"? Ask the community On the Prototype tab, in the Device section, select None or any other suitable device. https://help.figma.com/hc/en-us/articles/360039823894-Set-prototype-device-and-starting-point#Prototype_settings

Thank you for reaching out to the community! I noticed that you’ve already contacted our support team regarding this question.For others experiencing a similar issue, we can confirm that the Fit Width option is currently unavailable in the prototyping viewer. (This issue has been categorized as a feature request for resolution.) As a temporary workaround, our engineering team suggests using a device frame with a width of 500px.We’ve submitted a Voice of Customer request based on your feedback. However, please note that we cannot provide a specific timeline for when this feature might be implemented. Thank you for your understanding!

Hello!On the prototype tab, chose “Custom size” as your device, and put it as your desktop frame size, in portrait mode. You should be able to have different device sizes on a same file, just use different pages.Hope this helped!