Hi boybi,
You'll be happy to know that you have an option to reduce the size of your company file.
When you upgrade and convert your company file to QuickBooks Desktop 2019, the file size will not automatically reduce. However, you can use its new condense utility feature which gives you an option to remove the Audit trail information to date. The Audit trail information, on the other hand, contributes a great percentage on the company file size.
Before you proceed, I would recommend creating two backup copies of your file: one for the conversion and one for safe keeping (for your record).
You can refer to theses articles:
If you need anything else, don't hesitate to go back to this thread.