Pulling force is very complex and depends on the flux distribution in the region between the magnet and the steel plate that it is attracting.
Even with magnets of the same diameter and different length the leakage flux pattern will be different. The only case where pulling force might be remotely useful is with identical sized magnets of different grade.
For this sort of work, just forget pulling force. Neo characteristics are known and you can deduce all you want from the dimensions and grade.
The total flux that determines voltage is Phi and is B x A where A is the magnet area.
B depends on the rememant Br, which is directly related to the grade of neo and the ratio of air gap length to total magnet length. Br is typically 1.2T depending on the grade. When the air gap length is equal to the total magnet length Bgap should be about .6T but will be less due to leakage flux. With wider gaps the leakage increases rapidly. With gaps less than total magnet length the value of Bgap is linear so if the air gap is 1/4 magnet length then the Bgap will be half way between .6 and 1.2T
Summing up, Total flux in the air gap depends on the magnet area and the length of air gap. With round magnets doubling the diameter gives 4 times the area. Doubling the length will let you work with an air gap twice as wide for the same flux, giving you room for wire of twice diameter and 1/4 resistance.
Flux
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