There is not any maximum size set for the media files uploaded by rtMedia by default. However, if you want to limit the size for media uploads you can once visit this addon - https://rtmedia.io/products/rtmedia-restrictions/
And what is the difference between ActivityPlayer and SinglePlayer?
ActivityPlayer is the media player displayed under BuddyPress activity field.
SinglePlayer is the player which displays individual media files ( in a new page or in a lightbox ) uploaded by rtMedia.
Let me know if you still have any doubts.