For most chart types Microsoft Graph is used to render think-cell charts. Microsoft Graph natively supports chart sizes up to 327.55mm × 327.55mm. When editing a Graph object directly, without using think-cell, the chart is actually scaled in PowerPoint to the desired size if it is enlarged beyond this limit. You can verify this by inserting a Microsoft Graph chart of the same size:
think-cell does not support scaling with a factor other than 100%. Thus think-cell charts that are based on Microsoft Graph are limited to 327.55mm × 327.55mm. It is possible to drag the think-cell selection frame to a larger size, but the chart size itself will not increase. Charts that are not based on Microsoft Graph, e.g., Mekko or Gantt charts, do not have this limitation.
Note that PowerPoint's default slide width for widescreen formats is just a bit wider than 327.55mm. Hence, do not make affected think-cell chart types wider than such a slide.
Select the chart and reduce the size of the think-cell selection frame, using the resize handles as usual.
If you require the larger chart size, you can create the presentation on a smaller slide size with the same width-to-height ratio. To reduce the slide size, go to the Design tab on PowerPoint's ribbon.