I'm using Skybox beta version 0.2.3.3 which has the new subtitle size options.
Whenever I'm watching a 3D movie in the flat screen mode, the subtitles are only visible when the menu is open (so the video is playing in 2D in the background, and the subtitles are slightly visible from under the menu).
Whenever I'm watching a 3D movie in the curved screen mode, the subtitles are visible, but they are in the middle of the screen. The subtitles seem to get smaller and higher on the screen the bigger you make the screen.
Because of this the subtitles can be in the middle of the screen, and much bigger than the same subtitle size setting on the flat screen.