Relationship between bacteria size and temperature?
There is a negative correlation between growth temperature and genome size for prokaryotes, which may be driven by selection for smaller cell size overall in warmer environments. Sabath et al., 2013 provide a good discussion:


  Smaller cells tend to have smaller genomes in both prokaryotes and unicellular eukaryotes (Shuter et al. 1983; Lynch 2007), for reasons that are not fully understood (but see Dill et al. 2011). This correlation suggests that cell and genome size are functionally related, such that an evolutionary change in one would induce an evolutionary change in the other. Previous studies have hypothesized that reduction in cell size could be advantageous, either to optimize surface-to-volume ratio for uptake of scarce nutrients (Dufresne et al. 2005; Giovannoni et al. 2005; Moya et al. 2009), or to decrease predation by bacterivores or bacteriophages (Yooseph et al. 2010). Similarly, we speculate that bacteria adapted to high temperature could be subject to selection favoring smaller cell size, which also requires smaller genome size—regardless of gene content.