The color of dot tells you the relative size of the dungeon. Brighter is larger: so the orange dungeons are the largest.
I really hate to disagree with you, but I've found that the color/brightness of dungeons has more to do with their type/name rather than their size, as follows:
Darker Red: Cemeteries/Crypts/Graveyards (all very small)
Lighter Red: Ruins (all sizes...i.e. 2 to 4 modules)
Dull Orange: Castles/Towers/Strongholds/Guards (all sizes)
Bright Orange: Everything else (all sizes)
Other than the MQ dungeons their are none larger than 4 modules. The only way to find their true size (except for graveyards) is to play like Riki-Ticki-Tavi, run and go see.