We currently don't have a hard limit on the polycount or file size of imported 3D models, but it is important to keep performance in mind as you're authoring Enduvo content.
There are many factors aside from the 3D model size that can affect performance including the audience's hardware, the other assets in your scene, the textures/materials on the 3D model, and the virtual scene in which the content is being viewed.
Generally, 3D models with >2.5 million triangles will cause a noticeable performance drop, and you should shoot for well below that to account for scene objects, tools, other 3D models, and hardware differences, but this number varies quite a bit. If your content focuses on a single 3D model, it's recommended for that model to have around 500,000 triangles or less in PC-based content.
If your 3D model does not seem to be performing well, a technique called decimation can be used to reduce the polycount. Most 3D modelling applications offer decimation tools, but it can sometimes produce unwanted results. Both Meshlab and Blender are freely available software that offer this functionality.