Just a question about the scale you can use in blender.
Suppose i want to model a room with furniture. I want my furniture to be the same scale so it can fit into my room.

How do i do it? Can i for example use 1 blender unit for 1 meter or 1 cm? Will this impact performance?
Suppose that i take 1 unit for 1 cm and i have a sofa of 120cm50cm60cm. This way i can easily model the correct size in blender.
Will it impace memory/performance if i make my model 1205060 or

What is the best practice for modeling based upon real world values?

What is the best practice for modeling based upon real world values?
If you are not using the physics simulation, an external renderer, export to another program or subsurface scattering then just use an appropriate scale for your object. If you are modelling something that is 1km long then 1BU = 1cm would not be best. Anything using very very small lengths or very very large sizes can cause difficulty with the visibility in the 3d view.
Whatever scale you use also set the appropriate view and camera clipping end and start values (properties panel, keyboard N and camera settings) so you can see all of your object

Thanks. And what if i am using the physics sim or external renderer? Currently i don’t but i think i will be using it in the future so maybe it’s best i do i correct from the first time…

1BU = 1m for physics simulation

So it’s best, if i want to use my models in future scenes, i take that scale.
So in my example i should use for my sofa.

Remember you can easily scale your scene later. Just model it at whatever scale you find comfortable.