Most upper quality bike saddles give you the width and length measurements. Then you can compare that to your sit bone measurements. That will let you know if the width is ok, then you just have to know if you ride better on pear shaped seats or "T" shaped seats. (I don't know of any easy way to figure that out, other than by riding each style and seeing how each feels)
There is a thread in "Favorite Saddles" that talks about how to measure your sit bones. If you've already measured them for your narrow/medium english saddles you are ahead in the game!
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