Thanks for the report. There isn’t a limit on the size of structs, and it’s a bug that you get that error.
We’ve fixed one bug already that triggered this error for large optional structs (e.g. ?my_large_struct), but that fix is not yet in the public release of Fortnite. I believe that’s what you’re seeing here, but can you confirm that you’re hitting this bug for a large optional struct, and not simply for a large struct?