How to slice a spritesheet having sprites of varying sizes?

I see a lot of sprites packed with different sizes and I dont know the best way to slice them. For example the sprite sheet downloaded from here: https://0x72.itch.io/16x16-dungeon-tileset

is a 16x16 sprite sheet. But not all images in the sheet follow these dimensions, for example the attached is how it looks when I ask sprite editor to slice in size of 16x16.

As can be seen, other than the small characters the fountains, the ogres, the weapons are not sliced properly.

So how does one slice a sprite sheet like this properly?

Thanks