NMOS/PMOS Characterization
Always use the same transistor width and length what you would like to use in your circuit. If you do not know yet, start with your best guess.

Using minimum length will reduce your threshold voltage. But also would a minimum width on an STI process. Once you have a fairly high channel length, it will not vary any more. Unless you design an RF circuit, it is better to use longer channels than the minimum to reduce noise and mismatch.

Layout dependent effects could have an impact on your design, but I guess by default the model assumes that you have enough dummies. So I would not worry about that.