I tested to upload a pbix more than 600Mbs and it was uploaded without problem. I have no idea on the exact pbix size limit but I guess it is not less than 1Gb as in Power BI Service.
In my case, the network is good and uploading 600Mbs is just in minutes. So you could try upload in a better network. AKAIK, there's no log feature, I've submit a request for log in the git library, you can add you comment there.