Smaller and smaller.
A reverse extrapolation of the Gigantic GEN1 from the Build the Enterprise web site. Although it seems possible at first glance, it may yet fail in the details (where we know the devil resides ;-). It has the pleasant particularity that it is considerably cheaper than 1 trillion dollars. The payload to Mars orbit, however, is only a few hundred grams….at best.
Assembling it would give those nice astronauts in the International Space Station something to do on their long weekends….