Does Printf support the "I" size spec?

It supports %I64... - but why would you use that instead of the Windows functions?
I don't understand. Do you mean wprintf() (which supports "%Iu")? I've always thought that Printf(), and friends, were THE way to output in a plugin. How do you handle outputting a size_t which is 32 bits on x86 system and 64-bits on x64?
The Windows functions may behave differently if e.g. output is redirected to stderr.
If you need to you can wsprintf() to a buffer, and then QPuts() or Printf() that.
Yes, that's what I figured Printf() and friends were all about.
I use wsprintf() a lot. But I don't think it has a size/type spec that's automatically 32 or 64 bits as would be needed by a size_t parameter. OTOH, swprintf() does, but using it, or anything in the printf() family, adds about 16KB to the exe/dll.