You definitely need to have special ones taken that meet size requirements. Our DMP wanted the same immigration sized photos that we were including with our application, so we just had additional copies of those made when we got them taken. Others have reported that the DMP insisted on passport sized photos, which are different from immigration sized photos, but I suspect that the DMPs insisting on passport sized photos don't realize they aren't the same as the immigration photos.
To be safe, and for convenience . . . just find out how many the DMP requires, and don't ask about size. Get additional copies of the ones you have done according to the size specified in the CIC instructions for photos. It's only logical that since the photos are going to CIC, attached to a CIC medical results form, to be matched to your immigration file, they should be the same as the ones you provide to CIC with your application.