To reproduce:
In the attached project

1) Select the file Screen01.ui.qml.
2) Comment out the width and height of the item rectangle1.
3) If the Formeditor does not update immediately, mouse-click into it.

Expected Result:
The Item resizes to a width and height of 200 since this is the size specified inside the component.

Actual Result:
The Item vanishes since it has a width and height of 0.
This can also be seen in the PropertyEditor.

Solution:
To resize the item to its expected size, press the "Reload View" button in the Formeditor.
The Formeditor and PropertyEditor will update and show the expected size now.