How to reproduce:
1. Open the attached "WorldSpace" project 
2. In the Project window, open the "GameObject" Prefab
3. In the Hierarchy, select the "UIDocument" GameObject
4. In the Inspector, enter any longer number (4+ symbols) into the Width/Height under the "World-Space Dimensions"
5. Observe the results

Expected results: The number can be easily entered, same way it is when editing the fields not in a Prefab move
Actual results: It is hard to enter more than one symbol, as recompilation starts and the field becomes unfocused

Reproducible with: 6000.2.0a5, 6000.2.0b10
Not reproducible with: 6000.2.0a4, 6000.3.0a2
Can't test with: 6000.1.10f1 (World-space UI not available)

Note: Even though world-space UI was not released in the first 6.2 alphas, the already existing world-space panel settings seem to work