Instagram recommends using 1080px by 1080px and that is what we recommend as well. If you go lower, your image might come out blurry.
FYI - we do not downsize images if it is lower than 1080px by 1080px. However, if your image is larger, we will downsize it to 1080px by 1080px which is what Instagram does.
If you're doing a split image, please make sure that each image comes out to 1080px by 1080px. For example, if you're doing a 3 x 3 split, the original image should be 3240px by 3240px (1080px times 3).