Shapespark doesn't have explicit scene size (triangles) limits, but large scenes can fail to load on some devices (especially mobile phones) or can run with a low frame rate. Large scenes can also take a long time to download over a slow Internet connection.
It is recommended to keep the number of triangles in the scene below 4 million for it to work well on older mobile devices. If the scene is to be viewed on a desktop computer, and the loading time is not a critical factor, then the limit is much higher - in the 10-20 million triangles range.