LME is about 10% heavier than the same amount of sugar's worth of DME; so you can divide by 1.1 to convert from LME to DME. If your LME can is 3.3 lbs, use 3 lbs of DME, for example.
As for what kind, I know there are dark DMEs available, but at my LHBS they only have the pale stuff from Briess. If yours has something darker I'd go with that. Briess and Munton's both have a Dark, and Briess has Extra Dark which you can get from Northern Brewer. If you're using specialty grains (and I hope you are) the best thing would be to achieve the darkness and depth of flavor with that, and if the DME you buy is less flavorful than the LME you're replacing, you can up the specialty grains to fudge the flavor. A little chocolate malt goes a long way.