I'm sorry, I've been unclear. Again, I don't want velocity to factor in here.
What I'm looking for is to have particles orient in the direction of their velocity _and_ be able to independently control their X and Y scale.
In this instance, my Billboard mode is set to VelocitySpheroidalAlign, my AxisScale is set to 0 (I don't want to change shape relative to velocity), and my Size is a float2 (1,0.1). It's rendering squares with a size of 1.
The expected behaviour is that it would render rectangles with a size of (1,0.1), orienting the particle in the direction of motion. Are my assumptions incorrect?