The choice to prevent people from re-sizing their activities is to save you from yourself 
Generally speaking if the name of your activity has gotten so big that it doesn’t fit in the box it’s a bad name. A core practice of using BPMN is to ensure that the model is easy to read and understand quickly. if activity names are succinct and the symbols are uniform the model is demonstrably easier and faster to comprehend.
If you need additional information on your task - BPMN has annotations and cawemo.com lets you add comments and documents to tasks.
I hope this helps you understand that our decision is the right one.