Hey sorry for the late reply.. at the moment the tool doesn't really support that because the tool itself is a small resolution. It was kind of built like that by design - generally when stacks are too large the engine slows down.
I don't have the time to make such a change (as it involves more than just lifting the limit) but if you're interested, feel free to poke around the source code if you want to try modifying it yourself. It's using Godot 4.0: https://github.com/jontopielski/Stacked-Sprite-Viewer