Climate change is intensifying atmospheric demand for water in forests while altering soil moisture availability. How tree size modulates responses to water dynamics remains poorly understood in North American temperate forest species, including sugar maple (Acer saccharum Marsh.). We examined size-dependent tree water relations in an eastern Canadian forest by monitoring sap flux density (Fd, cm³ cm⁻² h⁻¹), stem water content (StWC, %), and tree water deficit (TWD) in 29 sugar maples across 2023–2024 summers as part of a throughfall reduction experiment. We analyzed how responses of these indicators to vapor pressure deficit (VPD) and soil relative extractable water (REW) varied with tree size (height, diameter, crown area, canopy status). Atmospheric demand strongly governed tree water relations. Large trees were more sensitive to VPD than small trees. Increasing VPD by 45% was associated with a ~40% increase in Fd for large trees and a 30% increase for small trees. StWC declined more steeply in large trees, with VPD-driven losses occurring ~30% faster than in small trees. Stronger water-use and storage responses produced double the increase in TWD in large trees, even though mean TWD remained lower than in small trees. REW exerted weak effects but interacted negatively with VPD: under drier soils, VPD-driven increases in Fd and TWD were amplified. Large trees maintained relatively stable TWD across soil moisture levels, consistent with deeper water access and short-term buffering via stem storage. These results show how tree size mediates coupling between atmospheric demand and water status in sugar maple.