I just moved my tub to make room for a concrete slab. I've never done a concrete slab, but the quickcrete widsite gives a good tutorial. Anyway...
I plan to extend my deck out about 10' and build a cabinet around the tub on one half of the new patio area. (daigram below)

The distance is 7' from lip to lip. It seems like the slab should be at least that big, I won't need that much to support the footwell.

Does it need to be bigger? or Should it be smaller?

Thanks.

Well, I decided on a 7'x7'x4" slab. This will give me a solid foundation for the cabinet, and I'll build a deck around it.