Yes, HEF files have a fixed input size.
Resizing should be done on the host. Since you are running on Windows I would expect your system to have enough performance to do this easily. Maybe you can find a better library.
Resizing on the Hailo-8 is only useful in rare circumstances. You are wasting PCIe bandwidth by sending images that are larger than required. So, unless your host is very weak and has enough PCIe bandwidth this is not the best option.
You would have to create a network e.g., in PyTorch that does not modify the image. I do not know whether it is possible to create this type of network and then convert it with the Hailo Dataflow Compiler.
You can try ChatGPT. It can probably create a simple network in PyTorch for you.