my answer didn’t post… damned.
Ok, basically, the size of the SMS does not influence the SMS setup time, which is the same whatever the length of the SMS.
The SMS flow is :
1/ Random Access from MS
2/ Immediate Assignment from BTS
3/ subscriber signalling (authentication, etc)
4/ SMS sending
Steps 1 2 3 are SMS setup, during which MS cannot answer to paging AND MSC is not aware how to reach the MS other than sending paging on the LAC. But MS is not listening to Pagings anymore.
Steps 4 is when MS is known by the MSC at the cell level (MM state). If sending a long SMS, the step 4 is gonna last longer.