I'm looking for an adjective for whether a shape is as wide as it is tall, i.e, is a rectangle or ellipses, vs a square or circle. "Rectangular" and the like usually refer to an object's sharp/round angles, not its width/length ratio. Right now I'm getting by making comparisons along one axis ("foo is thinner", "bar's height [which is greater than its width] imports a sense of length to car [which is associated with bar]") but it's not clear I'm talking about relative length, or that only foo is a regular/even/1:1AR shape. Is there a direct way to refer to and compare aspect ratios?
@BrianDonovan gave the very appropriate oblong, if you're looking for a word sounding more common, elongated also fits the purpose—it can mean something that "was made to be longer than wide", but also means "long in relation to width".
About oblong note that it seems to qualify only flat objects:
A rectangular object or flat figure with unequal adjacent sides
The word describing an aspect ratio close to one without completely prescribing the shape is rotund:
Approximately spherical; round, orbicular
The word is oblong:
longer in one direction than in the other direction (Merriam-Webster, first definition)
The term aspect ratio is used in contexts other than imaging and displays. In aeronautics, for example, aspect ratio describes the ratio of the length of a wing to it's chord (width). In such circumstances it is quite common to describe a long, narrow wing (e.g. a sailplane) as having a high aspect ratio and a shorter or wider wing (e.g. the North American X-15) as having a low aspect ratio.
While this usage appears to be close to what you want to describe and is common terminology for people already familiar with aspect ratios in certain contexts, it may still require further explanation/definition if your target audience is not already familiar with the concept.
The term of art in mathematics is called eccentricity. For an ellipse this is defined as the the ratio of the distance from the ellipse's center to one of its foci to the distance from that focus to one its vertices. Here's a pictorial representation:
The term is applied to other conic figures, and less often to rectangles. From Differentiation of Integrals by Shunchao Long:
... (the eccentricity of a rectangle is the ratio of its longest size over its shortest size).
In the case of computer displays, aspect ratios wider than 4:3 are also called widescreen.
For image aspect ratios in general wide is also used, and in addition flat is applied for some motion picture formats.
High usually refers to resolution, not aspect ratio.