While SVG's can be a purpose of their own, websites, accessibility readers and much of what you need to account for with SEO, is based on the standard IMG schema.
Because of this, it's best practice to do what search engines expect, which is to use the established good old IMG schema and all its attributes.
As SVG matures, no doubt we will see more robust image schemas develop, but for now, keep with the regular IMG schema that accessibility devices and all browsers understand.
Based on the above
Always define the width and height attributes of an image. This will help avoid unnecessary repaints and reflows during rendering.
Google Page Speed
If you don't markup code correctly using HTML height and width properties and it has an impact on your site, Google and other search engines will penalise you in search results.
Media Embedding
Furthermore, your images may not display correctly when embedding in applications or social media.
Always specify both the height and width attributes for images. If height and width are set, the space required for the image is reserved when the page is loaded. However, without these attributes, the browser does not know the size of the image, and cannot reserve the appropriate space to it. The effect will be that the page layout will change during loading (while the images load).
Sitespeed: Do images require width and height attributes?
When you include the image dimensions, the browser can draw out the "containers" that will hold the images, reserving the space for them while they download. The browser can then go on the paint the rest of the pages CSS and objects around those "containers" without having to go back and redraw the whole page once the images have downloaded and their sizes are then known.
See technical specification on W3C.