On macOS, Preview.app is highly underrated for all it can do. It’s my go-to app for a lot of things — one of them being fast image resizing. Let’s say you have an image that is too big for where you want to use it. Preview can quickly cut it down.
Open the image in Preview.
Click Tools → Adjust Size.
On this dialog box, you can resize the image to whatever size you want. I recommend keeping Scale proportionally checked so that as you adjust the width, it automatically adjusts the height so it doesn’t distort the image.