In my experience of NodeMCU's they are a great piece of kit, but once you start adding devices to them like relays and with the inclusion of a power supply then they start to get big and messy. Unless you have loads of room behind the light switch I think you will struggle for space.
If your backbox for the switch is deep enough you may get a small relay inside but then you've got to connect the mains wires (usually 1/1.5mm solid drawn cable UK) so then this will make it a very tight fit too... it will just become a nightmare IMO, then more important than everything else there's the safety element of it to consider.
If you have a neutral at your switches would it not be better to change them to smart switches? If not, how would you power the 5v power supply? if there's no netral available would it be possible to get a neutral to them?
For me, because of the lack of a neutral wire I replaced my lamps with ZigBee ones, leaving the switch left in the on position I installed the Phillips Hue dimmer switch & plate which neatly fits over the existing switch, then these can control up to four lamps from them (as there's four useable buttons available) - these are all directly compatible with HEand works flawlessly.