Gem said:

First off, make sure you always always have credits on the image when you post anything on Horse Eden. To resize this piece, first you'd have to cut it into a square to the proportions work better, then I personally use this website.
https://compressimage.toolur.com

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Change the image quality to 100%, resize it to 300 x 300 in the appropriate boxes. You will almost always lose some amount of quality but smaller canvases do help a lot.