How big can cold-blooded animals get?
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:


Reptiles
Amphibians
Fishes (Pisces)


Reptiles:

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).

Others:


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)


Green Anaconda http://www.wallshq.com/wp-content/uploads/main/2013_01/green_anaconda-wide.jpg
Green Anaconda



Amphibians:

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"


Others:


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)

Fish:


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



Mola mola



Oarfish



West Indian Ocean coelacanth



Bowfin