The Size of the Oberth Class - U.S.S. Grissom from Star Trek III

The Oberth class was a small Federation starship used primarily by Starfleet and civilian scientists alike, as a short range science vessel, from the late-23rd to the late-24th century. The ship class is named after Hermann Julius Oberth a German physicist and engineer.

• The premise for all my deck calculations: a deck is 2.5m + 0.5m for EPS cables, power cables, ... = 3m height for a deck. 
• My calculations follow a purely mathematical approach. I do not compare bridge modules or disk heights with each other. I always pull in decks based on the rows of windows and then calculate a physically plausible room height.

From the TNG episode "Hero Worship" we know that the Oberth class has two decks in the disc. This is the well to recognize on the MSD (Master Situation Diagram).

 

screenshot by trekcore.com

This information results in the following side view of the Oberth Class:

schematic by TheLightWorks

The "original movie" Oberth class would have to have a length of 125m to be able to be converted into the TNG style. This requires a bit more space between the decks.

A TNG-version (with 24th century Jefferies tubes - 4m deck high) results in a length of 166.7m for the ship.

The fact that the Oberth class has a standard deck height can be seen well on the USS Tsiolkovsky's TNG style corridor in TNG "The Naked Now" and at the bridge of the USS Grissom. If the ship was only 120.4m long (= 395ft), then it would have a deck height of only 2.89m. Thus, there would be no room for the Jefferies tubes. 

ILM Size Chart by Andrew Probert

The length of 125m or 166.7m is also much more plausible for a crew of 80 members.