Answers to your question
"how many seats the Tardis has, along with their approximate dimensions and the name of the designer,"
1. No seats, or an infinite number of seats. The TARDIS has a room that has a door that leads into the TARDIS...
2. Probably a small park bench dedicated to the memory of Mrs.Trellis of North Wales. 6ft long, 3ft high, 2ft deep.
3. The designer's name would probably be unpronounceable.