It seems impossible to have reptiles the size of dinosaurs, just because they are really big! Did they have different systems of maintaining body temperature or maybe they weren't the exact type of animals that we today call reptiles?
Answer is quite simple as from @Alan Boyd link. They are cold blooded and thus, can go out for hunt in cold, they need to stay put till they get some prey.
So, it mainly depend on the temperature of the outside, I found this interesting paper on relation of body sizes and latitude.
Body sizes of poikilotherm vertebrates at different latitudes
Maximum sizes of 12,503 species of poikilotherm vertebrates were analyzed for latitudinal trends, using published data from 75 faunal studies. A general trend appears which may be summarized by the rule "among fish and amphibian faunas the proportion of species with large adult size tends to increase from the equator towards the poles". The rule holds for freshwater fish, deepsea fish, anurans, urodeles, and marine neritic fish arranged roughly in order of decreasing clarity of the trend). In general the rule applies not only within these groups of families but also within single families. In reptile groups, the rule holds weakly among snakes and not at all among lizards or non-marine turtles. Possible explanations include an association between small size and greater specialization in the tropics; the possibility in poikilo-therms of heat conservation or of some other physiological process related to surface/volume ratio; selection for larger size in regions subject to winter food shortages; and an association between large adult size and high reproductive potential in cold regions. Other suggestions can be advanced, but all are conjectural and few are subject to test. Global size - latitude trends should be looked for in other living groups.
Cite: Lindsey, C. C., 1966: Body sizes of poikilotherm vertebrates at different latitudes. Evolution: 456-465
Now lets compare some of the largest cold blooded Animals:
The smaller size of Reptiles give them more agility to hunt andsave them predators but some times when they are top of food chain they can grow as giants. Anacondas (Eunectes), gigantic snakes from South America, are undoubtedly the largest living snakes. The largest species, the green anaconda (E. murinus), likely only rarely exceeds 9 metres (30 feet) in length; nonetheless, persistent but unsubstantiated reports have been made of anacondas that are 12 metres (40 feet) long. The reticulated python (P. reticulatus) of Southeast Asia and the East Indies has been recorded at 10.1 metres (33.3 feet).
King Cobra(Ophiophagus hannah) 5.5 meters (18 feet)
Keeled rat snake (Ptyas carinatus), at about 3.7 metres (12 feet).
Nile crocodiles (Crocodylus niloticus) 6 metres (20 feet)
American crocodile (C.acutus) 4–5 metres (12–15 feet).
Marine leatherback sea turtle (D.coriacea) 2.7 metres (9 feet)
Galápagos tortoise (Geochelone nigra) 255 kg (560 pounds).
Komodo dragon (Varanus komodoensis) 3 metres (10 feet)
Green iguana (I. iguana) 2 metres (7 feet)
According to Wikipedia:
"The Chinese giant salamander (Andrias davidianus) is the largest salamander and largest amphibian in the world, reaching a length of 180 cm (5.9 ft), although it rarely—if ever—reaches that size today"
West African goliath frog, 30 cm (12 inches) 3.3 kg (7.3 pounds)
Caecilia thompsoni 1.5 metres (5 feet)
Axolotl(Ambystoma mexicanum) 30 cm (12 in)
Chinese giant salamander
African goliath frog
Axolotl (Small 12 inch)
West Indian Ocean coelacanth, (Latimeria chalumnae), is 80 kg (176 lb), and they can reach up to 2 m (6.5 ft)
African lungfish (Protopterus aethiopicus), 2 m (6.6 ft) and may weigh as much as 50 kg (110 lb)
Ray-finned bony fish (Actinopterygii)
Ocean sunfish (Mola mola) 4.3 m (14 ft) from fin-to-fin, 3.1 m (10 ft) in length and weighed about 2,300 kg (5,100 lb)
King of herrings or oarfish (Regalecus glesne) 6 m (20 ft)( The longest known king of herrings, which was hit by a steamship, was measured as 13.7 m (45 ft) long)
Bowfin (Amia calva) 109 centimetres (43 in) in length, and weigh 9.75 kilograms (21.5 lb).
European conger (Conger conger) 3 m (10 ft) and a mass of 110 kg (240 lb)
Lancetfish (Alepisaurus ferox) 2.1 m (7 ft) long and can weigh up to 11 kg (24 lb)
Houndfish (Tylosurus crocodilus) 1.5 m (5 ft) and a weight of 6.35 kg (14.0 lb)
Giant Tigerfish (Hydrocynus goliath) 1.5 m (5 ft) and 50 kg (110 lb)
Golden Dorado (Salminus brasiliensis) 1 m (3.3 ft) in length and weigh 31.4 kg (69 lb)
Tambaqui (Colossoma macropomum) 1 m (3.3 ft) and 32.4 kg (71 lb)
Giant barb (Catlocarpio siamensis) 3 m (10 ft) and a weight of as much as 300 kg (660 lb)
Muskellunge (Esox masquinongy) 1.8 m (6.0 ft) and 45.4 kg (100 lb)
Atlantic tarpon (Megalops atlanticus) 161 kg (350 lb) and length is up to 2.5 m (8.2 ft).
The Atlantic cod (Gadus morhua) grows to 2 m (6.7 ft) long and 96 kg (212 lb)
milkfish (Chanos chanos) 22.7 kg (50 lb) and 1.84 m (6.1 ft) long
Electric eel (Electrophorus electricus) 2 m (6 ft 7 in) in length, and 20 kg (44 lb)
Opah (Lampris guttatus) 2 m (6.6 ft) in length and weigh up to 270 kg (600 lb)
Source: http://en.wikipedia.org/wiki/List_of_largest_fish Source: Britannica.com
West Indian Ocean coelacanth