What you want to measure is the distance from the center of the highest string to the center of the lowest string, and you want to measure this distance at the "break point" of the string over the bridge or saddle. That distance gives you the bridge string spacing, which can then be used to help select a replacement bridge or choose a standard humbucker vs. trembucker pickup. You may find that, especially for fingerstyle, you prefer one type of bridge (I prefer wider Fender-spaced bridges in general). So if you find your right hand technique works better on one guitar vs. another I would check this to start.
How should I measure it? Ideally you'd use a set of digital calipers - the differences are slight and if you are using a ruler you may not get it quite right. If you don't already own a set of digital calipers, they can be had for relatively cheap and they are quite useful around the house once you own them. As an example of what I'm talking about please see the image below: