While it is 21 ft long and 11 ft wide one end measuring 38" is not usable making the centre only 15ft by 11ft and at the other end 4ft measures only 4ft

there is no perfect dimensions for an island. Try to organce space subdividing into smaller cabinets of 60x60 cms which is the max space for a structural division and works for seating area as well

There is no average height for the table and chairs.There're height standards for all the furniture. The standard height for the table is 30" and for the chair, it is 18". The depth of the table should be atleast 36". It might be a little broad say 42" but not more. The length if the table is defined with the seats required. The chair seat size should be 16" with the back height be 30" atleast. It might be more depending upon the design of the table.

you should take place your dinning table near to the all by which u can get a lot of space to decorate these place much more beautifully.