I don't see any direct way to do this but here's a suggestion for a hack that might work. Create an image that matches the output resolution you need; even simple image editors like ms paint will allow you to do this. Import this image to your drawing and align it with the top left corner of the drawing. Then click on the background and drag on the handle that shows up on the bottom right hand corner. After adjusting the size of the page to match the imported image, simply delete the image from the drawing.

If you're creating a drawing for print, include the dpi setting that will be used on the printer. For example, if you'll be making a print on an 8.5" x 11" sheet of paper at 300dpi, create an image that is (8.5 x 300)px x (11 x 300)px.

This can quickly become troublesome if you need output at multiple resolutions.

Short Answer

Google Drawings doesn't have a ruler but it has "alignment, snap to grid and auto distribution" (see [2]) features. Alignment and distribution guides are automatically shown when the user moves one of the objects near to other objects that could used as reference to calculate the guides measures.

Remarks

As far as I know it's not documented in the official help articles or they were removed while others still include links to them, i.e. "Snap to guides"

References

• [1] Docs editors help
• [2] Paint and create charts and diagrams with Google Drawings