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What is the diameter of your Newtonian primary mirror and secondary mirrorr (minor axis of latter)?
Why do you want an aperture mask?
You mention inner and outer diameter. This implies an on axis mask perhaps placed directly over the mirror to block the effects of an edge defect in the primary, e.g. turned down edge.
You would not use an on axis mask at the front of the tube as it would reduce contrast due to the secondary diameter now being a greater percentage of primary diameter. (Contrast is a function of diameter not area). The effect can be minimal to extreme depending on the obstruction.
An off-axis mask is typically used and is limited by the physical dimensions of components used. Typically a 10" telescope with 2" secondary can accommodate 4" off-axis diameter portal that clears the secondary vanes as well as secondary mirror providing 4" of clear aperture.
Again you have to ask why do you want a mask. If the primary mirror is of good quality then when seeing permits, full aperture even with the central obstruction of the secondary will outperform especially in terms of resolution and light gathering a masked telescope. The benefits to double star observing are real in that the diffraction spikes no longer 'hide' secondary stars from view if they happen to have a position angle causing them to be behind a spike. However resolution is still less than at full aperture.
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