Didn't get your exact question but SIB1 carries information related to a cell which is being allowed to access to the UE such as PLMN-ID,Cell-ID,TAI,cell barred information,cell selection information and scheduling information of the other system informations.
So some parameters are optionals and some parameter are mandatory in SIB1.
Cant specify the exact size of SIB1 but depend on ASN.1 encoder and value of the parameter number of acquired bits will be calculated.
For example CSG-Indication is a boolean so it will take only single bit to represent it,TrackingAreaCode is BIT STRING of size 16 but number of bits allocated for this parameter is based on the value as if value is 5 then only 3 bits are required to represent the Tracking AreaCode.
