Sprites in Sprite Editor turn Invisible and cannot be sliced

I've made my own Sprite Sheets before, and i've imported them into Unity without any problems, but for some reason i brought in some new sprites and when i open up the sprite editor to slice them, they are not there, the sprite just vanishes, in the console i got some of the following errors

--------------------------------------------------------------------------------------------------------------------------- RenderTexture.Create failed: requested size is too large. UnityEditor.DockArea:OnGUI()

--------------------------------------------------------------------------------------------------------------------------- Trying to read pixel out of bounds, reading (0, 0, 5520, 240) from (1108, 351)

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also in the inspector it gives me a warning that only POT textures can be compressed when Mipmaps are enabled, and that only Textures with a width/height with a multiple of 4 can be compressed to DXT5 format

do any of you know what has gone wrong? I am also a massive amateur creator so any help is appreciated