For women's clothing, "too small" typically means a size that does not fit comfortably according to measurements of bust, waist, and hips, which vary by brand and country. For example, US women's clothing sizes start at size 0 or XS, corresponding approximately to a bust of 33-34 inches, waist 25.5-26.5 inches, and hips 35.5-36.5 inches; anything smaller than this is unusually small and may not be commonly available in standard clothing lines.
If the question was referring to another context (such as physical or anatomical sizing), there are differing perspectives based on the context. For sexual anatomy, studies suggest vaginas can be about 4 inches deep on average, with important nerve endings in the first 1-2 inches, and sizes smaller than about 4 inches in a male partner are sometimes considered less typical though many women report satisfaction regardless of size.
Please specify if the context is about clothing, anatomy, or another area so the answer can be refined.