Larger is easier to read / split degrees, and good if engine is on engine stand. If swapping cams in the car, need the smaller diameter to clear the k-member / frame, and such.
Be careful handling and storing degree wheel so they stay flat. If they wobble it makes it hard to read also.
Simple cam check. set tapped dial indicator to zero in the base of the cam lobe. Bring tappet lift to 0.050" or whatever spec tappet lift you want to check.
If not interested in installed centerline, just set the degree wheel to Zero @ 0.050" Rotate to max tapped lift and record max lobe lift. Continue to rotate to 0.050" before tapped is back on base circle. Read the duration. If you set degree wheel at zero, then the duration from zero is the duration spec for @ 0.050".
If you want to know install centerline, and LSA, you will need to set the degree wheel to zero when the piston is at TDC, then do the math of adding the duration before TDC to the duration after TDC. Depending on the degree wheel, may need to convert ABDC duration by adding 180 for the duration from 0 to 180.