This button is enabled for new projects I've created in Xcode 27, but I haven't been able to see it in a not grayed-out state with any of my existing projects created with prior versions of Xcode.
What is the logic for when the Enter Resize Mode button is enabled in the Device Hub?
The 'Enter Resize Mode' should be enabled when you are interacting with an iPhone and an app is active. In addition, the app needs to be linked against the iOS 27 SDK.
Additionally, please note that resize mode is only supported on iPhone (and not iPad) simulators running iOS 27 or later.
If you've met all of those requirements and are still not seeing the button be enabled, please file a feedback report including a sysdiagnose from your Mac and a simctl diagnose archive, and include details about which simulator you were using when you experienced the issue.