Tile Palette

Hello, im very new to unity.

I downloaded some Tilesets from the asset store, but when i take the Asset? and drag it onto the palette, it doesnt fit.
I googled it and appearantly its because of the Pixels Per Unit is by defalt on 100px? I can make it fit by editing that number manually, but it cant be that i have to do this for 100s of tiles?

Also, how do i remove something from the palette?

there is and edit button in the palette