Scaling text in Keynote

I have a complex drawing in Keynote, which I need to resize. I can do this by making the drawing into a group, but unfortunately it contains many text labels, which do not resize when I change the size of the group. I can change the font size, but it's quite hard to get the new font size to look the same as the old one. What usually happens is that the new text is slightly larger than the text boxes, so I end up having to manually resize each one in order to make it readable again.

Usually I get around this by taking a screenshot and inserting it into the presentation as an image. However, in this case I need the image to have a transparent background. I tried using Keynote's Instant Alpha feature, but it left white in the interiors of letters like "O" and "a", and manually removing that is just as much work as manually resizing all the text boxes.

So is there some kind of method or workaround for resizing the text of a keynote drawing along with the other elements, while keeping the background transparent?

In Keynote.app: Select all parts of the graphic you need to scale, then copy them to the clipboard (Command-C).

In Preview.app, just create a new document (Command-N). Contents from the clipboard will be automatically inserted. Save this document as PDF somewhere, and drag it to Keynote. This should do the trick.

My solution:

• duplicate the slideshow,
• resize the entire document (Document tab of the 'properties sidebar' > Slide Size dropdown)
• copy the correctly sized elements back to the original doc.

Any variation of this will probably work - the key principle is that if you resize from the Arrange properties tab, objects will resize whilst text won't; if you resize the document: everything stays in proportion.

Using the accepted answer's method, I wasn't able to edit the content I'd pasted from preview (the pdf contains a rasterized png, I believe).