like mike said. although most short tube refractors used are of the ed80 type I have seen smaller scopes yet used, but the smaller the scope the longer the exposure needed. what you could do is buy an 8" or 10" dob and save up for the mount. an eq6 will handle both of them (not together obviously) and be quite stable.
I originally bought a 8" dob and it is now mounted on an heq5 (not to be confused with an eq5) which is about the limit for the mount. its fine for planets though and i could probably do DSOs too but thats not my bag
whatever you choose enjoy
ps: dont forget the cost of the camera and mounting equipment on top of the other costs...