Some recipes call for 6.6 or 3.3 pounds of LME because some manufacturer's sell that size can. If you want, you could substitute DME to make up the difference, or even use all DME. The only difference is to use a bit less of the dry (since the liquid has water in it, you use more), usually about 75-80% of what you used use in LME. For instance, for a recipe with 6.6 pounds LME, you could use 6 pounds LME and .5 pounds DME; or 3 pounds LME and 3 pounds DME. All would give you about the same SG.
(It's pounds of extract, not gallons!)