I would measure your existing front sight, and get one close to the same size, width and height. Your adjustable rear will give you some leeway height wise, width wise I would not go any wider, so you can have a noticeable amount of daylight on both sides of your front post, within your rear notch, to make it easier to "center". The fiber optic will allow you to hopefully focus on the front sight post.
Included is a list of Sig's common fixed sight "heights", check to see if yours has a small number stamped on it. This could alleviate you having to try and measure.
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