If you have an Apple machine, I’ve recommended here before the “Image Compress and Resize Easily” app, available in the App Store. It uses a simple slider bar that tells you as you move it just how many kB you’re at. A few minutes practice and you’ll have the knack. It’s so much simpler than others I tried, and I tried and deleted lots of photo editing apps.
I don’t know if it’s available for Windows machines or at Google Play for androids. I reckon one of these days I’ll have to check. But as Steve notes, Paint works well enough, though I usually reduce images once to 28% or so. As I recall there is no kB readout to tell you when you’re below 300 for sure, so if you have an original high MB image, it may take multiple reductions to get there.
Joel