Checking the size of the image nodes is difficult without the Godot editor.
https://itch.io/t/838806/core-tag-size-rules
I don't make image packs so I don't have many tips for making them. There is a channel on the Strive Discord that is dedicated to portrait packs(https://itch.io/t/284398/discord). The game only supports PNG, JPEG, or WEBP image formats. Most use PNG because they like to replace the background with transparency, though some feel that it is just extra work especially with the fading added by the Bugfix mod.
Something to keep in mind about image sizes is that anime images often have very fine details for things like hair or facial features, when those are shrunk to the portrait size they may not appear correct. Also, while there may be small benefits to using larger images and letting the game scale them down, this results in rather large file sizes. There is no benefit to increasing the size of images unless you have some kind of image processing to add detail.