What would be the best way for me to "slice" these using Unity's Spritesheet editor? As all of the elements in here are of different sizes.
I'm not sure I haven't really done too much experimenting with Unity, I wouldn't know where to start. Sorry!~
No worries :)
I suppose I should ask, what IDE were these assets designed to be used in?
Or is it possible for you to slice the assets into separate images?
I tried to make them as universal as possible so there isn't any specific IDE I had in mind, so in any image editing program or any IDE that's capable of it, remove the background and you should be able slice the individual images to make use of them!