Hi Maxime,
You can do this with ExtendScript: grab the active sequence's settings, change videoFrameWidth / videoFrameHeight, then apply them back with setSettings().
var seq = app.project.activeSequence;
if (seq) {
var settings = seq.getSettings();
settings.videoFrameWidth = 3840;
settings.videoFrameHeight = 2160;
seq.setSettings(settings);
}
Note that frameSizeHorizontal / frameSizeVertical are read-only, so setSettings() is the way to apply the change. Save the snippet as a .jsx file and run it via File > Scripts > Run Script File.
Reference (Premiere Pro Scripting Guide – Sequence object, documents getSettings()/setSettings() and the videoFrameWidth/videoFrameHeight properties):
https://ppro-scripting.docsforadobe.dev/sequence/sequence/