From the official Apple support document:

If you update to iOS 10 beta, you can remove some built-in apps from the Home screen on your iPhone, iPad, or iPod touch.

When you remove a built-in app from your Home screen, you also remove any related user data and configuration files. This can affect things like related system functions or information on your Apple Watch.

The apps built into iOS are designed to be very space efficient, so all of them together use less than 150MB.

The page goes on to list the apps you can remove from your iOS device.

Interestingly, some apps have a fallback and others do not. For example, if you delete the Calendar app and have Fantastical installed, Fantastical will automatically fill the calendaring role (you can add calendar events to Fantastical via Siri, for example). But if you delete the Mail or Calculator app, nothing fills the gap, even if you have alternatives installed. This is a beta, so that behavior might change.

[H/T Dr. Andrew Leavitt]