How do I know if I should use mesh edges, mesh face or mesh volume to create the mesh grid for a model? If it's a simple shape i guess it's more straight forward to use mesh volume but if let's say it's a room with other objects in it. How should I go about doing it? I tried using mesh volume but it seeems that some of the face are not mesh. Will appreciate any advice given.

It's better that you split the whole geometry into simple ones and the ones with more detailed structure. To mesh the one with small objects, it's better to mesh from the small volumes (or mesh some edges/faces to control the cells number).