Is there an official case size cutoff for what is and isn't an SFF case?
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The next largest case type, the mini-tower, is usually defined as a case that stands 12" - 18" tall. We could probably label cases that are shorter than 12" - 18" as SFFs? I couldn't find an exact size or volume cutoff for SFF cases, however. The general consensus seems to be a wishy-washy, "SFFs are smol."
SpoilerSources: Small Form Factor Buyers’ Guide by Anandtech, Types of Computer Cases and Motherboard Form Factors by Utilize Windows, Small Form Factor and Computer Case on Wikipedia.