Try building an onion rocket and a smaller payload. Or better, build a serial rocket and watch the lower stages get blasted by the upper! (That is always fun to watch)
*facepalm* That would be useful information, too... I think it will be around 1.8m still. Helldiver, may I request a fairing adapter (1.8m <-> 2.5m) to be mounted on the aft end of the shuttle for this purpose? Version 2.0 perhaps?