The size of the Reliant class

The Reliant class was a type of Federation starship operated by Starfleet during the early 25th century. Based on the configuration of the Miranda class from the 23rd century, it is based on the same construction method.

According to this blueprint the ship has 14 decks.

schematic by Star Trek Online

• 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.

With a deck height of 3m, this results in a total length of 355m.
The official length of 251m definitely can't be correct, which can be deduced from the rows of windows - especially visible on the bridge segment (decks 1 and 2). If the ship were 251m long, then decks 1 and 2 would be one, or the deck height would only be 2.12m including the cables and technology. This would leave only 1.62m of headroom.