"if they instead use a over-size fusor and use the walls as repositorium of the target(that it is itself used for heating some fluid to generated electricy later), that may be done very cheaply and probably will get positive energy output without a lot effort."
Ummm, do you know how fission works? (what our current nuclear power stations run on)
Or even fusion for that matter?
lets say you meant fission when you said 'fusor'. if you have an oversize reactor there, you will reach critical mass within no time at all, regardless of how many control rods you have. also, for heating up liquid, THAT IS HOW THEY WORK! the heat from the reaction turns water to steam, which is shoved through a turbine!
Lets say you meant fusion when you said 'fusor'. a tokomak requires all materials inside it to be in the plasma state. (Dictionary for dummies: plasma: A very HOT gas) if you had it up against the sides of the tokomak, it would melt, thats why they have to keep it in a magnetic field. if you had it any bigger, the magnetic field would have less of an impact, and would be less successful in keeping the plasma under control, which would touch the sides of the tokomak, forcing the entire operation to shut down.
P.S. please learn english, thats the language we speak here, not engrish!