No dimensions are given but we get to see people sitting in and standing around the craft. It existed as a full-size prop with the characters. We do not know the heights of these individuals, so a 6-foot tall scale man helps us get the length (excluding the probe sticking out the front, which is approximately two feet).
Supercar has on occasion been outfitted with weaponry, but this is not part of the standard configuration.
In Atomic Witch Hunt, pilot Mike Mercury states the speed that he is flying.
In Space For Mitch Supercar is converted for space travel and travels at 18,000mph. This is not standard. In Transatlantic Cable Mike leaves New York City and seems to immediately call base saying he will be there (Black Rock Nevada) in about 15 minutes. This is a distance of about 2,600 miles, meaning we could estimate that Supercar would be travelling at about 10,500mph. These have both been excluded.
Supercar is a unique invention. It has the ability to travel easily in the air, under the sea and on land. It can take off and land vertically and features several futuristic gadgets. On one occasion it even went into space with some minor modifications! Taken on what we see in the show, Supercar is indeed a marvel. Clearly without much basis in science, but we are basing this on what is seen and said in the show.
The year is not stated but it is commonly believed that Supercar takes place in the present day (when the show was made). Season 1 aired in 1961 and season 2 in 1962.