What version of Premiere Elements are you working with and on what computer operating system is it running?
What exactly do you want to do?
Each Premiere Elements project has a project preset which either you are the program set. There should be a match between the project preset selected and the source media that go into the project. There are numerous project presets to select from. Changing the frame sizes of the existing project preset in the new project dialog is not an option.
If your source video has a frame size that you cannot match up with an existing project preset, then we can talk about your alternatives to get the video into project.
And, once you have set a project preset, the party line is that it cannot be changed.
The goal of the project preset is to direct the program to established an appropriate space in the Edit area monitor from which to edit your video, typically without black borders. We can talk export settings opportunities after we iron out the import details.
Please detail your situation and what you want to do, besides just the generalized want to change frame size.
By the way, what is you intended export for this project?