tl;Dr: I'm trying to get a whole metabolic pathway into lettuce (Lactuca Sativa) via agrobacterium transformation. The pathway isn't the issue, but I'm trying to find out how many kb of DNA I can put into the caset and still get good results from the transformation?

Hi there! I'm writing a proposal for funding for an experiment where the end goal is a cultivar of red lettuce (Lactuca Sativa) which is able to produce ergosterol, or pro vitamin D. There are tons of mushrooms that make this compound, however this is a 21 step enzymatic reaction. In stead of trying to ID the genes in the fungi of choice and then optimizing the codons from there I was thinking of taking the sequence of the enzymes, turning those back into DNA while choosing a more optimized codon bias, and then adding exons as needed. Once that is done I was going to have a caset made with CMV promotor to be grown in agrobacterium and then use them to transform the lettuce. Only problem is I know 21 enzymes worth of DNA is going to be massive, and I don't know how I'll fit all of that into a caset. I was already thinking of doing multiple transformations but how many would depend on how big the caset can be and that really depends on how much DNA can successfully be integrated into the lettuce and THATS where I need your help! Also, if you have anymore pointers on experiment design, materials, or methods I am all ears! This is my very first project I get to take the lead on and it feels like I'm driving a rocket ship! Thank you tons for your help :)