Can this be used in bath bombs??

It is generally recognized as safe, so technically yes - It is a heavy fine black powder it will separate out of a bath bomb as it dissolves and settle to the bottom of the tub.

Could this be added to eyeliner to make it magnetic for lashes?

The powder is magnetic so theoretically it could be used for this purpose, it is, however, essential that the binder, contain the powder to prevent particles getting into the eye which would cause irritation

do you know range of particle sizes?

The avarage particlesize is 44 microns (mesh size 325) it is a very fine dust-like powder

silly question-approx how many Tablespoons in a bottle? making multiple batches of magnetic slime for a bday party..

1 Tablespoon = 1/2 fluid ounce : But since this is a very dense and fine powder it is very heavy for its volume This 4 ounce (By weight bottle) contains about 2 fluid ounces (Volume of product) so the correct answer is 4 tablespoons which would each weigh about 1 ounce.