Around the bathtub there is a large space. Caulking will fall right through.
The space is about 1 1/2 wide. I need some kind of backing first before I
Apply the caulking. Any ideas?

Are you saying that the tub is 1 1/2" away from the wall? Is this a new tub or a new wall?

around the bathtub there is a large space. caulking will fall right through.
the space is about 1 1/2 wide. I need some kind of backing first before I
apply the caulking. any ideas? this is an old tub. There is a gap
In someplaces between the tub and the wall.

They sell a product called backing rod it is like foam you can find it in home depot you out that in the caulk but I would try to look ender tub from below if possible and see why you are getting large gaps goo liuck