Teacup ray is a generic term that labels a number of small rays..
Reticulata, Hystrix, and Humerosa are your smallest rays, all reaching disc sizes of around 12-16".. However, that dosn't include tail length. Reticulatas get an extra long tail.
The fact you don't know the dimensions of your standard 125g would tell me that you're not ready to consider Stingrays.. The biggest battle you will face is water quality, because 125g is not alot of water volume and would require frequent massive water changes.. for a single ray.
IMO, 72x18" isn't sufficient for life for any stingray, unless you REALLY know what you are doing and have the right species.. even then, it is debatable.