Hi.
1 - Go to the Document Settings (Cmd/Ctrl + J);
2 - Set the desired dimensions;
3 - Check "Scale content";
4 - If the objects of your animation are not still in the size and position that you want, go to the bottom of the timeline and click the button Edit Multiple Frames (Shift + Alt/Option + E);
5 - Drag the handlers to select the frames you need to adjust;
6 - Select multiple objects from different frames at once with the Selection Tool, Lasso Tool, shortcuts, Layer panel or any other method;
I hope this helps.
Regards,
JC