On document properties, CTRL+SHIFT+D, yo can change the size of the page. Then you can resize in different ways:
1) Select all the objects and then resize manually dragging the transform points
2) Select all the objects and on the "select and transform object" context menu, F1, you can change the size introducing values. There is a "lock" picture if you want to resize with an aspect ratio. Then you only have to change it thinking what is the largest side of your graphic and enter the relative values with that side of your document's size.