I am currently working with a landmass roughly 1,000 miles (1600 km) in length and width. Would insular dwarfism or gigantism occur in this landmass? If not what is the largest a landmass can be before this does not occur.
There is a biometric scaling formula that deals with the size of animals (in kilograms) in relation to the area of their habitat (in square kilometres).
From this formula it is easy to calculate whether animals will undergo dwarfism in the habitat of any given size.
For both herbivores and carnivores, island size, the degree of island isolation and the size of the ancestral continental species appear not to be of major direct importance to the degree of dwarfing. However, when considering only the body masses of recent top herbivores and carnivores, and including data from both continental and island land masses, the body masses of the largest species in a land mass were found to scale to the size of the land mass, with slopes of about 0.5 log(body mass/kg) per log(land area/km2). There were separate regression lines for endothermic top predators, ectothermic top predators, endothermic top herbivores and (on the basis of limited data) ectothermic top herbivores, such that food intake was 7 to 24-fold higher for top herbivores than for top predators, and about the same for endotherms and ectotherms of the same trophic level (this leads to ectotherms being 5 to 16 times heavier than corresponding endotherms).
Source: Insular dwarfism
First, your landmass is gigantic and is by no ways small.
The reason for insular dwarfism is not the size of the landmass but the limited ecosystem and amount of food it can provide.
Small landmass is of course the primary reason for a limited ecosystem but you could also have dwarfism in a gigantic landmass if for some environmental reason, the ecosystem is very poor (think desert)