I've been fiddling with svg sprites a bit today, trying to handle them in such way that you could use any unit you like to specify their size. Here's what I came up with.
(You could use this method for bitmap sprites as well, but I don't see much sense in this!)
All images in the sprite must share the same width or height, preferably both. Note that maintaining rules for images with several different aspect ratios may be problematic.