formato grandezza

How do I change the video format/size of a Premiere project that has already started?

By @davide_8751

To change the size of the timeline you can click in it and then go to Sequence > Sequence Settings. If Frame Size is greyed out, change the Editing Mode to Custom.

The video format for the preview files can be changed in the same dialog. But, there is no need to do it unless you plan to render the timeline when you are working on it and then use the preview files when creating the final output. The video codec choosed there does not affect the output such as magically enhance the video quality from bad to fantastic. So change this only if you must.