Svg Image Tag Size

An ‘image’ element establishes a new viewport for the referenced file as described in Establishing a new viewport. The bounds for the new viewport are defined by attributes ‘x’, ‘y’, ‘width’ and ‘height’. The placement and scaling of the referenced image are controlled by the ‘preserveAspectRatio’ attribute on the ‘image’ element.

So the solution you're looking for is to set the width/height to the maximum area you expect an image to fill and then set preserveAspectRatio to the appropriate value to set the scale and position the way you want (the spec provides an example SVG showing some of the possible behaviours for preserveAspectRatio).