UE information (UE capability, number of ERAB,,,) and eNB RRC Configuration (Data Forwarding, number of rrc measurement ie, SIBs,,,) are the variable size parameters in the handover message. So you can't calculate correct size.
Following is some handover signalling messages size. (ERAB number = 1)
S1AP Handover Required : 366 Byte
S1AP Handover Request : 419 Byte
S1AP Handover Request Ack : 219 Byte
S1AP Handover Command : 214 Byte
X2AP Handover Request : 409 Byte
X2AP Handover Request Ack : 206 Byte