How do I get the size of nlohmann object in memory in bytes?
The .size() operator gives the number of elements.
I have a lot of sub keys and elements.
What can I do to find it?
Technically, you'd get this by using a custom allocator. The NLohmann JSON library supports custom allocators, which replace std::allocator. You can provide one that counts the various allocations. Note that the support is a bit flawed.
But as the comments suggest, you want a smaller representation. The working copy in memory is typically bigger, fragmented, and even if you collected all the fragments there's little you can do with them.