Hard drive constraints and operating system for 2009 13" macbook I was given a 2009 13" macbook, but it needs a new hard drive and operating system.

What is the most current OS it can handle and how much space does that OS take when freshly installed?
Your MacBook can support anything up to and including High Sierra (the next OS release), but will not have Handoff due to the computer not supporting Bluetooth 4.0.
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As for the install space, lifewire’s article says that you should have 16GB of space minimum for the install, though 32GB is recommended.