In selecting a size for your angle head, the largest head capable of accessing the cavity or bore and making the required cut is generally the best choice, in terms of durability and rigidity of the head. The larger the angle head, the larger the bearings, gears and tool holding capacity.
The exception might be if you anticipate the angle head will not be dedicated to the specific application you are considering at the moment and the nature of your work tends to be smaller. If a smaller head would be more valuable to you overall and still capable of accomplishing the task at hand, it might be a better long term choice.
Angle head size selection can be determined by the MBS (Minimum Bore Size) and MTC (Maximum Tool Clearance) as given in the Key Cutters Sizes and Dimensions section of the website. However, we can sometimes "shave a little off" a head, so give us a call if moving up to the next largest head size is marginal.
Next Subject: How to Choose Angle Head Style.