A set of simple scaling relations is derived to assess the impact of plasma physics and technology assumptions on the design of a DEMO tokamak fusion reactor. At the same time, it is shown that by postulating that the plasma physics assumptions are consistent with those that can be reliably reached in present-day experiments and that the recirculating power is reasonably low, a tokamak DEMO operating with steady-state plasma operation is of large size, comparable to a reactor - suggesting that the study of pulsed options should receive more attention in the future. The scaling relations reproduce well the results from a number of previous studies, indicating that they are particularly well suited for future parametric scoping studies. From the relations derived, it also follows that the areas in which future progress will have a particularly large impact on the attractiveness of DEMO are the limit in plasma physics and in technology the magnetic field strength Bt and the wall-plug efficiency CD of the systems to drive noninductive current.