"This is a big bathroom, praise be to God!"
Translation:هٰذا حَمّام كَبير اَلْحَمْدُ لِله!
They can be used interchangeably most of the time, when it comes to spaces. However, to be precise:
- Big: كبير (kabír).
- Spacious: واسع (wási3).
So, I can say that كبير is more general than واسع in some sense, since it can be used for sizes, volumes, as well as areas and spaces sometimes. On the other hand, واسع is kind of restrict to spaces and areas.
Imagine a room, with wide area but a low roof - I think you can describe it as spacious but not as big - Just how I see it.