The short answer is to use less polys in the models.
The download size of the app is about 15-16 MB
All the chairs appear to be cloned in a single FBX despite all being the same mesh. What would be better here is to have a single chair in the FBX and clone it in the scene 5 times. That should reduce the download size for the chairs.
The walls shouldn't need to be anywhere near 24,480 triangles so that can be optimised WAY down by the artist.
The characters also have a high poly count so again the artist should look for ways to reduce that.
Once you have created the cubemap, you do not need to load the 6 individual images so untick preload on them.
In terms of appropriate size, it's more about getting the user to something they can engage with the content as soon as possible. So you need to keep the initial download size as small as possible to get them into the experience and if need be, load more assets in the background.
The overall size needs to be appropriate for the application, so for example a 20MB package size for a simple Tetris game is too large but for a Sega Rally like 3D game, it be 'fine'.
Taking http://tanx.io/ as an example, the title screen and options load within 2 MB and the rest (about another 14MB) loads after.
Virtual Voodoo https://www.miniclip.com/games/virtual-voodoo/en/#t-sd the apps loads the customiser and title screen first so they can interact with the content and then blocks the user from starting the game till all the assets have been loaded.