General Motors Corp. rolled out the only upsized model of the year here today - its new RTS bus that weighs 26,000 lbs, or roughly 5 per centmore than the 1959 model it replaces.

The price tag on the striking, sleek and modern-looking city bus also is upsized: $80,000 to $90,000 depending on the options.

Not included among the options are air conditioning and a rear window. Air conditioning isnt optional because isn't standard: a rear window isn't available because that space is entirely filled with the air conditioning unit.

Robert W. Truxell, general manager of the GMC Truck and Coach Division said the bus represents an investment of about $50 million in facilities, equipment and tooling plus "many millions more in engineering and testing."

For dramatic appearance, the 13-ton bus probably topped most of the new 1978 model cars. It has smokey windows made from acylic, not glass, that give the vehicle a dark sunglasses-like appearance in daylight.

When the bus stops to let passengers on and off, it automatically, "kneels" down at the right front corner with a hiss, lowering the entry doorstep a full five inches to less than 13 inches above the pavement.

The exterior fiberglass body panel is acrylic-coated and nearly impervious to spray paint graffiti. It's designed to be replaced by segments to reduce maintenance down time.

The rear door is extra wide and was designed specifically to accommodate an optional hydraulic wheelchair life.

The floor of the bus is 22 inches above the street surface during operation, compared with 32 inches for current transit buses.

Truxell said the RTS "is designed to attract new bus riders."

Gerald Haugh, general manager of the Long Beach, Calif., transportation company, came here to accept delivery of the first RTS. Haugh said he expects "the exciting look" of the RTS to attract people who don't normally ride buses. Whether they will become regular rides is another question, he said.

Truxell estimated that GMC will sell about 4,000 RTSs over the next year while another 1,000 buses of the current design will be sold by GM's competitors.