Size: 300 x 600 nm
The reason why genitalium has such a small genome is that parasites tend to have smaller DNAs. So it must be highlighted that genitalium can only survive in highly enriched environments, it can't even make its own amino acids, which it normally obtains fromthe host cells! And because it cannot do cellular respiration, it very likely replicates slower than say E. Coli. It's easy to be small in such scenarios!
Power, Sex, Suicide by Nick Lane (2006) section "How to lose the cell wall without dying" page 184 has some related mentions puts it well very:
One group, the Mycoplasma, comprises mostly parasites, many of which live inside other cells. Mycoplasma cells are tiny, with very small genomes. M. genitalium, discovered in 1981, has the smallest known genome of any bacterial cell, encoding fewer than genes. Despite its simplicity, it ranks among the most common of sexually transmitted diseases, producing symptoms similar to Chlamydia infection. It is so small (less than a third of a micron in diameter, or an order of magnitude smaller than most bacteria) that it must normally be viewed under the electron microscope; and difﬁculties culturing it meant its signiﬁcance was not appreciated until the important advances in gene sequencing in the early 1990s. Like Rickettsia, Mycoplasma have lost virtually all the genes required for making nucleotides, amino acids, and so forth. Unlike Rickettsia, however, Mycoplasma have also lost all the genes for oxygen respiration, or indeed any other form of membrane respiration: they have no cytochromes, and so must rely on fermentation for energy.