There are no standard measures of fungal fitness, and yet descriptions of natural selection in fungi require an understanding of how to compare the success of two individuals. Success, or fitness, is normally understood to be a combination of survival and reproduction. Xanthoparmelia cumberlandia is a sexual, lichenized fungus. By recording the size of, and number of sexual structures on, individual lichens we demonstrate a significant correlation between size and reproductive effort in this species, showing that size is an easily measured surrogate of fitness. Published data of other lichen species, for example Umbilicaria spodochroa or Xanthoria parietina, also show a correlation between size and sexual fecundity, indicating that the correlation may be a general feature of sexual lichens. However, patterns of resource allocation differ between lichen species. Published data collected from U. spodochroa are linear, demonstrating that larger lichens allocate equivalent resources to growth and reproduction. In contrast, the data of X. cumberlandia are curved, indicating that in this species larger lichens allocate a disproportionate share of resources to reproduction.