This paper examines macro and micro-level patterns of genome size evolution in the Brassicaceae. A phylogeny of 25 relatives of Arabidopsis thaliana was reconstructed using four molecular markers under both parsimony and Bayesian methods. Reconstruction of genome size (C value) evolution as a discrete character and as a continuous character was also performed. In addition, size dynamics in small chromosomal regions were assessed by comparing genomic clones generated for Arabidopsis lyrata and for Boechera stricta to the fully sequenced genome of A. thaliana. The results reveal a sevenfold variation in genome size among the taxa investigated and that the small genome size of A. thaliana is derived. Our results also indicate that the genome is free to increase or decrease in size across these evolutionary lineages without a directional bias. These changes are accomplished by insertions and deletions at both large and small-scales occurring mostly in intergenic regions, with repetitive sequences and transposable elements implicated in genome size increases. The focus upon taxa relatively closely related to the model organism A. thaliana, and the combination of complementary approaches, allows for unique insights into the processes driving genome size changes.