Need help to understand texture size, texel density and large buildings workflow.

As the topic says, i struggle to understand the best way to modeling large buildings and what texture size i need, also what texel density size i should use per cm or meter.

I don’t know if i should make buildings modular, or as one big mesh. The problem i find with a big mesh is that the quality of the texture get’s blurry when baked if the building is to large,
again with modular buildings i would get better texture quality, but also alot more textures.

I see that people recommend to tile/reuse the textures, but if i paint the textures or not, i would get some kind of pattern.

How do you know what texel density to use?
How do you know what texture size to use?
What is the best workflow for large objects such as buildings?