Is there a term to describe a shape that has uniform dimensions?
Squares and circles would fall into this category for 2D shapes, as they are as wide as they are tall. Rather than ellipses or rectangles, since these can be wider than tall and vice verse.
Similarly, cubes and spheres would have this attribute for 3D shapes.
This term would also be quantitative. Some shapes would have more or less of this attribute.
Symmetrical comes to mind, but that doesn't seem accurate. For example, the shape of a tree is symmetrical, but often much taller than wide.
Cheers
The phrase "uniform dimensions" is very vague, confusing, and open to numerous interpretations.
I think you're seeking Meissner bodies, of which Reuleaux triangles are the most famous example.
See also here.
As a mini-tutorial on how to ask better questions, if I understand your question, here would be a better way to state it:
What is the mathematical term for a convex shape in $\mathbb{R}^n$ that spans the same maximum distance in one direction regardless of the orientation of that object?