To reproduce:

1. Open the project, attached by the tester (ArraySizeBug.zip)
2. Select MyObject and MyObject1 assets in the Project browser
3. In the Inspector observe that arraySize is 3 instead of 2, which is the minimum size

Expected: arraySize shows the minimum size as it is stated in the documentation: " If the SerializedObject contains multiple objects it will return the smallest number of elements" (see https://docs.unity3d.com/ScriptReference/SerializedProperty-arraySize.html).

Reproduced in 2017.1.3p4, 2017.2.2p4, 2017.3.2f1, 2017.4.1f1, 2017.4.2f2, 2018.1.0f2, 2018.2.0b3, 2018.3.0a1