One of the biggest problems with the Federation ship classes is that a larger size does not mean better combat performance. Examples of this are the Galaxy and Sovereign classes. The Galaxy ships are bigger but Sovereign class ships have twice as much space devoted to weapons than Galaxy class ships do. So, for the purposes of the STM, Sovereign class ships should be considered larger than Galaxy class ships.
Galaxy-Class
Development Project Started: 2343
Production Start Date: 2353
Production End Date: Still in Production
Current Status: In Service
Length: 642.51 meters
Width: 463.73 meters
Height: 195.26 meters
Weight: 4,500,000 metric tonnes
Hull: Duranium microfoam and tritanium plating
Number of Decks: 42
Sovereign-Class
Development Project Started: 2365
Production Start Date: 2373
Production End Date: Still in Production
Current Status: In Service
Length: 685 meters
Width: 224 meters
Height: 88 meters
Weight: 3,255,000 metric tons
Hull: Duranium-tritanium composite with micro-fiber reinforced ablative armor.
Number of Decks: 24 total, 22 habitable.
From ACTD - Advanced Starship Design Bureau
http://techspecs.acalltoduty.com/index.html