I think the mini fan advice is about right.

The problem with small scale jet pipes is that the wall friction and viscosity effects sap power seriously compared with full size, where a tapering exhaust is used to increase gas velocity. Also we are talking about just a fan, whereas full size gets energy from hot gas.

So the rule of thumb is to build an inlet as smooth as possible, keeping the area of the fan, and an exhaust should be not much longer than the fan diameter and only slightly tapered. Picking a 'fat' subject with a short tail pipe, like many of the early jets with non axial engines seems a good idea, otherwise you could stretch the fuselage diameter or move the fan towards the rear.

With modern combat jets at small scale perhaps the best solution is to use a pusher propeller, not too visible in flight and more efficient than a fan.

By the way there has been some discussion of tandem fans, but it seems the consensus is that the extra weight does not give any gain because of even worse efficiency - has anyone tried it or is it just a theory?

good luck

Jerry