List available printers or use default printer?

[rjc2013]

Listing the available printers is more of a Windows-API question than a Rust question (I'm assuming you're on Windows).
You would need to call the EnumPrinters function in the Windows API. Here's a thread where someone had to do that from Rust (HIH):

Windows-rs and raw point to array of struct

Hmm... I haven't looked very close at EnumPrinters, but here's my guess at what's happening: The PRINTER_INFO_2 buffer contains a bunch of null-terminated strings. It's the responsibility of the application to allocate the buffer. This is me just speculating, but these two facts point to that the reason you're getting such a large "needed" buffer is because you need to allocate enough for all the PRINTER_INFO_2 buffers and all the strings it will point to. EnumPrinters will store all the PR…