IIRC, once you establish a font size it will remain the same...regardless of how you group then reduce your graphic. What you may want to do is to group the shapes, then insert the text at the appropriate font size and re-group it all together.

On another note, you may want to use other software (PowerPoint, Visio, et al) to create your graphic to your specifications, then import the file into your Word file.

Hope this helps...