The most recent Power BI weekly updates ( Sept 22, 2015) added some much needed improvements to the available list of tile sizes. Until this release, there were only a small number of sizes available.
While the introduction of Power BI 2.0 has been well received, the limited tile sizing was a presentation drawback, This release greatly expands tile sizing flexibility that will give report designers a greater ability to deliver the reports being requested.
In this weeks release, it was also suggest that an update to Power BI Desktop will be release very soon. For more details go to the Power BI Weekly Service Update report.