The pictures in the Miniatures Handbook are the actual size the miniatures will appear - or very close to it.
The biggest Dragon is likely to be the Large Red Dragon figure (young red dragon) pictured in the MHb. It stands about 9 cm tall on a 40 mm base. (A little under 4 inches).
Compared to the standard figures, it is HUGE. However, it is only a Large figure under 3.5E rules; Huge and bigger figures cannot be produced in the standard line as they don't fit in the boxes... there is a strong possibility that they will be produced in special sets, though.
All miniatures in the D&D Miniature line are meant to be scaled properly to each other. They occasionally have mucked up, but the designers have indicated a desire to improve the scaling issues.
You can find a few other details about Dragoneye (including a partial set list) on my website. See my sig.
Cheers!
