Instead of searching for an answer based on size, which habitats/climates would fit your story best? There is actually plenty of possibilities of adaptations to temperature within reptiles of the size your looking for. I will mostly look at the lower temperature border in this answer.
Have you considered mesothermy? Mesotherms are animals who because of high metabolic rates generate enough heat to have a constant body temperature despite changing surrounding temperature. One extant example is the leatherback turtle, who has even been found within the Arctic Circle! High muscular activity, counter-current heat exchange, thick fat covering and incredibly something similar to brown fat tissue found in mammals for generating heat without shivering.
But even more extreme were Dinosaurs! There has been a lot of debate about the metabolism of dinosaurs and some scientists believe that some speceis were in fact warm-blooded. You can find a lot about that in this exhausting wikipedia article. 701
Some dinosaurs species have lived in very cold conditions and the article gives some temperatures:
Dinosaur fossils have been found in regions that were close to the poles at the relevant times, notably [...] the North Slope of Alaska [...], so polar dinosaurs and the rest of these ecosystems would have had to cope with the same extreme variation of day length through the year that occurs at similar latitudes today (up to a full day with no darkness in summer, and a full day with no sunlight in winter).
Studies of fossilized vegetation suggest that the Alaska North Slope had a maximum temperature of 13 °C (55 °F) and a minimum temperature of 2 °C (36 °F) to 8 °C (46 °F) in the last 35 million years of the Cretaceous (slightly cooler than Portland, Oregon but slightly warmer than Calgary, Alberta).
The possibility of migration is disregarded:
But a round trip between there and Montana would probably have used more energy than a cold-blooded land vertebrate produces in a year; in other words the Alaskan dinosaurs would have to be warm-blooded, irrespective of whether they migrated or stayed for the winter.
So to answer your question, maybe something around 2 °C (36 °F) can be the lower limit for year round human-sized reptiles. With hibernation you can even have it colder: the farthest north extant terrestrial reptile is the Viviparous lizard although you will have size-constrains for your human-sized reptiles...
For the upper limit, similar approaches are possible. If you want your species to live in hot deserts, think about heat exchangers like the neural spines of Spinosaurus, bright skin colour, behaviour adaptations like night activity or borrowing/living in caves. Compare for example here. 671