Dimensioning for an SMC connection is fairly straight forward. An sms can have a max of 70 16bit characters, or 140 8bit characters, or 160 7bit characters which translates to 140bytes max per sms. So, if you know your BHSM (Busy Hour Short Messages), you can easily come up with the number of signalling links you need, e.g.
No. Of Links = (BHMS * 140 * 8 * 2.5) / 64 000
where BHMS is your Busy Hour Messages (an estimate), 140 is the max length of each sms in bytes, 8 is for converting bytes to bits, 2.5 is to cater for the 40% loading, and 64 000 is the speed/size of each time slot on an E1 (64Kbps).
I hope that would give you a rough picture.