There is no single file size limitation — it depends on a few variables.
In theory, the only limits would be memory and performance. Currently we recommend keeping your files around 200mb, on the high end, but sometimes an extremely high poly count (~1.5 - 3 million) can affect performance as well.
Our team is here to help make sure the asset import process works for you, and we are more than happy to test your models and/or get you on an efficient workflow using the models you create. Just hit us up!