Easiest to plot it on paper yourself. Preferably to scale
Start with internal diameter, add depth of casting, add depth of preferred insulation, add depth of external covering ie render or brick.
That very simplistically tells you the minimum size you need. But then it’s down to you to factor in an allowance for space around it, how much worktop you need. Whether it has a long landing area to the front etc etc.
All assumes of course that you have no constraints that will prevent it being over a certain size (fence location, size of your patio etc)
If of course you mean depth of foundation rather than size then a slab of minimum 4” depth is plenty for most things assuming it’s properly laid over compacted hardcore. Adding rebar is probably overkill on most projects but you can’t decide to put it in after so use it anyway for peace of mind.