A good resource for SMR's is the " Catalyst Handbook" by Martin Twigg. It has quite a bit of design information and worthwile trying to pick up.
In my experience, the small "can" type reformers ( up fired, up flow) generally have a small number of large 5"+ tubes. The larger down flow reformers generally will go with tube between 2.5" and 5" depending on output and catalyst volume. The more exotic materials also may help the wall thickness / heat transfer rate / Creep life. A micro alloy tube may function better than a HP or an HP modified Tube.
I don't know that this helps but it may get you started.