When posting images on social media, size really does matter.
Each platform has optimum sizes for various images from profile images to feed pictures, the point is to make them just right to keep your followers coming back for more.
One of the easiest 'wins' you can make across social channels is to maintain consistency, not only in terms of the content you share but also the sizing of images.
Keep in mind that people read a lot of material on their phone, so make the images a size that can be viewed on the phone. These dimensions should accomplish that.