Invest In The Literature
Before you lay yourself open to the salesmen, read the chapter in Wills (Mineral Processing Technology) regarding comminution. As far as I recall it mentions that larger rolls will give a better nip & explains that advantage in mathematical terms, which is always a plus. Larger rolls will deflect less. Capacity is largely determined by face width & peripheral speed. The feed size is also important, which goes back to the nipping issue described by Wills & in other texts about mineral processing.
I bought my first Wills in Jo'burg in 1979(first edition) & bought the seventh edition in Perth in 2007 & found the content a bit changed but not all that much. There was an error, easily spotted & solved, in the sampling chapter of the first edition & I forgot to check it out in the seventh. All in all that's not bad for engineering texts where proof reading is almost non-existent.
Enough digression, just read Wills, or similar & you can't go wrong. 
But also remember that crusher manufacturers hope to generate most of their income from spares!
Now watch the sales blurb roll in.
■
Roll Crusher Capacity and Specs
Hello all,
We are intending to purchase a roll crusher for a limestone application where it in required to have minimum fines and maximum 2-6 mm product.
We were offered many models with different rolls diameters, widths and rotation speeds.
It looks that the higher the speed, the smaller and cheaper the rolls crusher will be.
The cheapest roll crusher that I was offered was the fastest one with 300 RPM speed and it was the of course the smallest physically.
Is there a disadvantage in having a higher RPM, regarding the product characteristics?
Is there an advantage in having higher rolls radius?
How does each of the parameters: radius, width and RPM effect the capacity and the product characteristics?
Is there a rule of thumb for the Roll crusher capacity?
Amit ■