In image view | editing mode, click button Resize on Editing Panel, a dialog allows you to choose the new size for the image,
1. Input the new size of the image;
2. The option of Contrain the proportion is checked by default;
3. Choose the sampling filter you want to use.
Click OK and if you want to save your editing, click button Save in the toolbar on the top.
To undo or redo your editing, please click the tab Editing History on the bottom of Editing Panel, all your editing history is listed and click one of them to roll back.