dear
for SDCCH dimension there as I know there are two ways two make dimension for SDCCH .
1- it depends on number on TRX for example from 1 to 2 TRX put one SDCCH/8.
2- from 3 to 4TRX put 2 SDCCH/8
3- from 5 to 6TRX put 3 SDCCH/8.
or there is onther efficient way that you have to know the ratio TCH traffic to the SDCCH traffic which acceptable value is between 10 to 40 % also you have to take consideration half rate penetration in to consideration.
hope it helps you