9) MBBR carrier surface area and other characteristics.
10) Future expansion.
11) Oxygen transfer rate.
12) Biodegradation of the organics.
13) Others.
Now about the MBBR, yuor need know many other factors as BODu, kg O2/h that your system can dissolve, and some others.
Thinking in the optimum conditions including having a good equalization system and constant flow to the treatment system, some authors could recommend for your system:
1) Grease tank volume: 5 to 10 m3 to be on the safe zone.
2) MBBR volume: 60 to 200 to be on the safe zone.
This is not mandatory, but it will depend on the evaluation of all the mentioned factors and some others.