3" is the way to go. Also, if you are committed to using a straight, it is worth spending extra to get a good one. I think of the price beginning at $75 for a good one. I have a Straight Razor Designs, 20 inch, English Bridle Leather with a nylon webbing strop as well and couldn't be more happy. It was about $95.
One caution. I would encourage a less expensive strop when you first begin. You will cut the first one to pieces. Whipped Dog has a little starter strop that is hard to beat for that. You could also get one of Straight Razor Designs less expensive latigo leather strops and replace the leather with good English Bridle Leather once you find your groove.
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