I want to make a model of some of the colossi, can anyone tell me how large each of the colossi is? Height, width, depth.
In order of appearance:
Valus is roughly 70 feet (21M) tall.
Quadratus is about 98 feet high, and 115 ft long from head to tail.
Gaius is 97 feet tall.
Phaedra is 98 feet tall, and about 90 feet long.
Avion is 140 feet from beak to tail with a 130 foot wingspan.
Barba is 80 feet tall.
Hydrus is 280 feet long.
Kuromori is 16 feet tall, and only 56 feet long.
Basaran is 75 feet tall, 160 feet in length.
Dirge is 260 feet long.
Celosia is tied as the smallest colossus at only 12 feet tall and 18 feet long.
Pelagia is 89 feet tall, and 100 feet long.
Phalanx is absolutely massive (the largest colossus in fact), clocking in at 557 feet long, with a wingspan of 200 feet.
Cenobia is the other smallest colossus at 12 high and 18 long.
Argus is 75 feet tall.
Malus is 195 feet tall - roughly double the height of any of the other 'upright' colossi.
While I don't have a direct answer to your question, I was able to find this comparison picture.
I imagine that if you could somehow come by the game's art book, from which that picture is taken, you'd have it at a higher quality.
Since I suspect your models won't be to scale, then a relative comparison might be sufficient in the stead of absolute measurements.
After following the link provided by Aubergine I found the de-facto number from the ICO team itself. 70 feet/21 meters tall. In the trivia section of http://teamico.wikia.com/wiki/Valus
To add to Aubergine's answer, it appears that the whole artbook is on Picasa. So, here's a bigger rendition of the same comparison picture.