How to calculate len size from unsigned char* in C++20?

Yes, unsigned char, signed char, and char are all exactly one byte large, so prores_ks_trellis_node_comp_glsl.size() gets you the size in bytes of your string.

The subsequent two lines are unnecessary, and it's also worth noting some problems:

std::u8string(prores_ks_trellis_node_comp_glsl.begin(), prores_ks_trellis_node_comp_glsl.end()).data() is taking the pointer to a temporary char8_t array, so that pointer immediately dangles. Also, calling the constructor of std::u8string like that doesn't do any transcoding. Your original string already needs to be UTF-8, otherwise this doesn't make much sense. There is no reason to use std::ssize instead of std::size here. You're trying to get a std::size_t result anyway, and std::size does that directly.

Assuming your goal is to "get the UTF-8 bytes" of the std::string, the proper way to write it looks like:

std::string s = "..."; std::span<const unsigned char> ut8_bytes( reinterpret_cast<const unsigned char*>(s.data()), s.size() );

This obviously assumes that your string literals are UTF-8 encoded though. If not, you'll need to transcode s to UTF-8 before taking the bytes.