Single-pulse all-optical switching (AO-HIS) is emerging as a key pathway to ultrafast and energyefficient magnetic control, yet its microscopic stochasticity remains concealed in continuous films that exhibit apparently deterministic reversal. Here, by patterning Gd-dusted Pt/Co/Pt into disks from 1.5 µm down to 400 nm, we show that lateral confinement exposes a fluctuation-dominated regime in which the switching probability exhibits an exponential dependence on disk diameter under a single 35 fs pulse. Combined with micromagnetic simulations, the measurements reveal a competition between a spin-transfer–induced bias, originating from ultrathin Gd, and intrinsic magnetization fluctuations during relaxation of a strongly demagnetized Co layer. Within this framework, reversal proceeds through stochastic domain-wall–mediated reorganization, and the diameter dependence is quantitatively captured by a single bias parameter that encodes interfacial angular-momentum transfer. These results establish size scaling as a direct metrology of ultrafast switching stochasticity and provide general design principles for achieving deterministic optical control through interface engineering and nanoscale confinement.