You have absolutely nothing to worry about. This plane is the last version of the MD 80 series made

When Douglas merged with Boeing (under the Boeing name), Boeing just changed the name of the plane from MD 95 to Boeing 717.

The whole line of DC-9 and MD-80/90 planes varied in size, but the size differences are not significant to passenger.

The DC-9-15 was 104 long.

The DC-9-20 was 119 feet long.

The DC-9-40 was 125 feet long.

The DC-9-50 was 133 feet long

The MD-80 series planes are 147 feet long.

The MD-95/Boeing 717 is 124 feet long, right in the middle. If you boarded a MD-88 and a MD-95/Boeing 717 you would not notice any different in size.