Hydrophobic confinement allows to supercool liquid water below its temperature of crystal homogeneous nucleation, making accessible to experiments a thermodynamic region that cannot be entered in bulk water. However, it is debated how the confinement affects each property of water when compared to bulk. Here, we investigate, using Monte Carlo simulations and a coarse-grained model of water, the low temperature phase diagram of a water monolayer confined between hydrophobic plates. By defining the appropriate order parameter and performing finite size scaling, we show that the model has a liquid-liquid critical point (LLCP), between two liquids with different density and energy, that belongs to the universality class of the the two-dimensional Ising model. Surprisingly, upon decreasing the size of the system, the universality class approaches continously the three-dimensional Ising model, as it has been found in bulk water. Therefore, our results lead to the unexpected conclusion that the smaller the confined monolayer, the closer the critical behavior to bulk water at low temperature.