Exporting .emf (Enhanced Metafile) vector from Illustrator CS5 [and other versions tested] for use in MS office documents such as PowerPoint, can be a right pain.
If you export them at a sensible size, the vectors don't have enough points and you end up losing accuracy on your shapes and strokes - so they look very low quality.
If you make them bigger in Illustrator before exporting, the filesize remains approximately the same, but the vector accuracy is far better. But when you import into PowerPoint you have to shrink them down, which is frustrating.
I searched for a solution for hours, downloaded all the metadata editors I could find, but eventually worked out an easier method to resize / change resolution of the emf file - answered below...