When it was Excel file earlier, it was pretty much easier to find the exact size of data model using the method given below
How to Find Power BI Excel Data Model Size. But how to find the data model size in case of power BI designer file? Do size of pbix represents model size ? Power BI free says a data capacity limit of 1 GB. So this represents total size of pbix and other files uploaded on Power BI service. Does data model size really matter ?
Yes, it's the size of the .PBIX file that cannot be more than 250MB if you want to upload it to PowerBI.com. Number of rows is irrelevant. The file also cannot have more than ten reports. These are the two main limitations.