I'm testing the effect of soil treatments on root and shoot length of seeds of a species. However the seeds had different sizes, which may affect root and shoot development. I have a measure of the area (in cm²) occupied by the nut in the seed that could be considered as a proxy for seed size. I'm thinking about using the nut area as an offset in the Gamma models. I'm not sure if this would be adequate nor in which unit the response variable will come out. It would be of no use if the response come as" length per seed area", which I believe it would be the case when using the offset. Should I consider using area as a fixed term in the model or is there anyway I could account for nut area as offset and back transform the results to length units?
Edit: I'm using log(seed area) as an offset term.