Requirements for choosing the number of observed variables

Estimation
1

Dear Prof Pfeifer,
My model is better identified (and more stable) for example with six observed variables.
When I increase the number of these variables, identification and stability deteriorates.
It is normal?
What are the requirements for choosing the number of observed variables(except for the connection with shocks)?
Please.

Thanks a lot.

2

See the discussion of the topic in “Remark 7 (Which Observables for Estimation?)” of Pfeifer(2013): “A Guide to Specifying Observation Equations for the Estimation of DSGE Models”