What is the actual width of the seat and what is the height of the seat?

The actual chair dimensions on the Comfo-Back 3 pc Bar Set are found under the specifications tab on the webpage and below.

Dimensions    Chair: 28 in w x 31 in d x 51 in h, Arm height: 35 in
Overall Width    Chair: 28 in 
Overall Depth    Chair: 31 in 
Overall Height    Chair: 51 in
Seat Height    26.5 in
Seat Size    20 in w x 18 in d