Hmm, my last reactor (in 1.7.10, BR 0.4.0rc) was 8x6x6 and output about 15 kRF/t at powerProductionMultiplier=1.0. Scaling that up, 1-2 MRF/t are realistic if you consider that the core goes from 4x3x4 to 21x21x23.
(BTW, maxReactorHeight is 48 in the standard config, maxReactorSize is 32.)