The upload limit is a function of the website code that does the upload. You can upload a 64mb ZIP file, which could be much larger inside.
Neither and both. The previews (and all the 3dtools) are voxelized presentations. Your model is converted to voxels (little tiny cubes), and then a decimation (polygon reduction) algorithm is applied to the resulting mesh to reduce the total number of triangles required to display the model. It's that (reduced) version which is presented to you for review in the browser, but it's not "real".
When you download the model, you only get back your original model. But.. your ORIGINAL mesh is sent to the printer.
To be hyper-technical, the printer itself also does it's own form of a voxelization, based upon the diameter of the nylon powder granules and the width of the laser beam. But.. the printer doesn't apply a decimation algorithm, other than simply the effect of physics and the resolution of the printer.
Power tip: Even if your model has no serious errors, run the ThinWallsFix, and save the result. When you download THAT model, it'll be the voxelated/decimated model. The statement is that the voxelization is done at a level below the resolution of the printer, so it SHOULD not affect the final print.