I presume you've used a torque wrench Thomas? How would you increase torque with one of those ?
First apply more force (muscle) to the end of the wrench. To do that with a steam engine you would up the steam pressure and increase the area (diameter) of the piston.
How else can you increase torque with a wrench? Double the length of the wrench. In a steam engine that equates to a long stroke.
So in your high torque engine you want high pressure, large diameter bore and long stroke.
If you think about it, with a steam engine you can get maximum torque with the engine stationary, all you need is for the angle between the con-rod and crankshaft to be 90deg.
But like most things in life steam engines are a compromise.