Dear librarylady: In theory, a lumpectomy can be performed on any size breast. The question is whether ALL of the affected tissue can be removed, leaving enough tissue for a good cosmetic result. Sometimes, small breasted women fare better with mastectomy and reconstruction because adequate tissue removal may leave undesirable distortion. If the disease is multifocal (more than one cluster) a mastectomy may be recomended to be certain that there are no other "yet unidentified" areas of concern. As far as the "enhancement," there is no reason why you could not do this. There may be some concern regarding screening of this breast. You should discuss this with your breast surgeon as well as your plastic surgeon.
Lumpectomy can be done in most cases; if the surgeon thinks there will be significant distortion, then it might not be the best choice. However, the way biopsies are done nowadays (not closing the hole left by the removal) distortion is minimal to absent, even in a small breast. But if there's more than one area, simple mastectomy is a consideration for therapeutic as well as cosmetic reasons. And yes, the opposite side could be enhanced to match an implant on the treated side.