Count: 0 — No reptiles were found that meet the specified criteria. You sought a global, species-level list of reptiles whose common names begin with the letter U, each entry verified with scientific name, habitat and size. Consulted authoritative sources such as The Reptile Database and IUCN produce no standardized, widely accepted common-name entries that begin with U, making this letter empty for the requested, strict format. Note that U is one of the least common initial letters in English common names for reptiles, which helps explain the gap.

Explain the absence: taxonomic and naming practices drive most reptile common names toward descriptive or geographic words (green, desert, tree, African) or toward translations of Latin genus names that do not start with U. Many genera do begin with U, but their accepted common names use other words (for example, Uromastyx are usually called spiny‑tailed lizards; Uroplatus are leaf‑tailed geckos; Uma and Uta species are known by descriptive common names like fringe‑toed or side‑blotched lizards). Exclude extinct, doubtful, or poorly documented taxa, and rely on standardized databases, and the strict search still yields no qualifying entries.

Consider near-misses and next steps: expect to find genus names that start with U in scientific lists (Uromastyx, Uroplatus, Uma, Uta), and occasional informal or locality-based English names that begin with “Uganda” or “U‑” in field guides, but these are inconsistent and not accepted as standardized common names in major databases. Consult The Reptile Database or IUCN and search by scientific genus starting with U if you need entries related to the letter U; otherwise expand the criteria (allow genus‑name usage, locality names, or include taxa with U in the scientific epithet) to produce a usable list.