Add note about VM size availability

Author: ntrogh

articles/lab-services/quick-create-connect-lab.md

@@ -51,6 +51,8 @@ Follow these steps to add a lab to the lab plan you created earlier:
     | **Virtual machine size** | Select *Medium*. |
     | **Location** | Leave the default value. |
 
+    Some virtual machine sizes might not be available depending on the lab plan region and your [subscription core limit](./how-to-request-capacity-increase.md). Learn more about [virtual machine sizes in the administrator's guide](./administrator-guide.md#vm-sizing).
+
 1. On the **Virtual machine credentials** page, specify the default **username** and **password**, and then select **Next**.
 
     By default, all the lab VMs use the same credentials.