The electrochemical CO2 reduction reaction (CO2RR) has the potential to mitigate the rising CO2 levels while storing renewable energy in chemical bonds. Copper is the only single metal electrocatalyst producing high energy dense hydrocarbons, albeit into 16 products on polycrystalline Cu. Therefore, more selective catalysts need to be developed. Inspired by single crystal studies, well defined metal nanocrystals (NCs) can leverage controllable surface facets and high surface/volume ratio to improve activity and selectivity. This thesis aims to use colloidal Cu NCs with tunable shape and size to firstly demonstrate the importance of these features to direct CO2RR selectivity, and then leverage the facet dependence of Cu NCs to expand on the knowledge regarding mechanistic pathways to high value CO2RR products. Firstly, the known selectivity of Cu{111} to CH4 is investigated at the nano-scale by synthesizing octahedral Cu NCs (Cuoh) with different sizes. The major Cu{111} facet directs CO2RR selectivity to CH4, and the size-dependent performance reveals that the ratio between Cu{111} and {110} edges determines the promotion of CO2RR over the competing hydrogen evolution reaction (HER). The smallest synthesized (75 nm) Cuoh show an overall CO2RR selectivity of 77% with a CH4 faradaic efficiency (FE) of 55%. A contemporary topic of debate is whether the high value product C2H5OH forms via *CHx-*CO coupling or *CO-*CO coupling. As a platform to address this dichotomy, we use CH4 favoring (*CHx populated) Cuoh and C2H4 favoring (*CO-*CO populated) Cucub under elevated *CO coverage induced by Ag NCs. We show that at the expense of CH4, Cuoh selectively promote C2H5OH, while Cucub make both C2H5OH and C2H4. We propose that C2H5OH formation proceeds via *CHx-*CO coupling, and the preferred sites for this coupling are the {111} faces on Cuoh and {110} edges on Cucub. Hence, we posit the counter-intuitive phenomenon of a CH4 selective catalyst becoming C2H5OH selective under CO saturation. Additionally, we suggest that catalysts having larger defect-to-terrace ratios are expected to be C2H5OH selective under CO saturation. We then proceed to tune the Cucub edge/face ratio by size control, as smaller Cucub are expected to possess increased selectivity towards C2H5OH by virtue of their higher proportion of edge sites. Indeed, smaller Cucub show larger C2H5OH/C2H4 ratio under CO saturation conditions further strengthening our hypothesis that Cucub edge sites are C2H5OH selective via the *CHx-*CO coupling pathway. This thesis showcases the strengths of colloidal NCs to reveal structure-sensitivity relations for which the achieved size control (i.e. importance of facet ratio) proved crucial. Furthermore, the possibility to use shape-controlled NCs as a platform to investigate reaction pathways is proposed. The mechanistic insights provided here could aid catalyst design targeting high selectivity for valuable CO2RR products.