In previous AX version, once we want to change the string field size, we just only edit its String Size easily. But it's funny that there are more steps in Dynamics 365.
Scenario:Extend invoice field to 30 characters.
Overlaying or extension:Not matter I try both overlaying and extension. The String Size properties is always disabled. Only using EDT overlaying, that allow to change the Extends properties.
EDT overlaying is available, only when the EDT isn't in Application Platform package. As below, the InvoiceId is in Application Suite, it's allowed to adjust Extends properties.
So, your project must be in the model that under ApplicationSuite package for overlaying the EDT.
Then, I create a new EDT with String Size 30 and replace it in InvoiceId's Extends properties. Now the String Size is changed to 30.
Later that, don't forget to do full synchronize database (Go to Dynamics 365 > Synchronized database) to check if there is an affect with some object. If there is an error, you must adjust that object too.