HoH:S (p. 59) describes the use of props in ceremonial casting, but some of the details are still unclear to me. First, do you receive the listed casting score bonus for each prop used or just the biggest prop used? If it's the former, is there an upper limit defined anywhere or does it need to be HR? Also, the text says using props makes the process take more time, but no actual rules are provided for this. Anyone have any guidelines they're using?
The examples given don't say how the bonuses stack up, but prop use is part of Ceremonial Casting, so magi are already getting their Artes Liberales and Philosophiae added, so I would say one bonus based on the largest prop (so +1 to +5) is a suitable bonus which won't break the game.
Props can make casting take longer, but seeing as they are used as part of Ceremonial Casting, which takes 15 mins/magnitude in an unprepared space (down to 1 min/magnitude for a Mystic Choreographer in a prepared space) I'd say the extra time can be considered part of the Ceremonial casting time (but maybe add a few minutes on if a Mystic Choreographer keeps wheeling out giant statues every time they are casting a low-level spell)
I think the bonus listed are for the total size of your prop collection: if you can fit them all into your belt pouch, that's +1, if you need a wagon to move them around, that's +4, and so forth.
Of course, each prop should also correspond to a specific Art, so maybe the bonus should be for the biggest prop you actually use for a specific casting, but unless you like keeping tracks of dozens of individual items I'd recommend just going with a 'prop collection', and assume it covers all Arts. After all, for a magus, the cost of a handful of gemstones isn't that much of an issue, but encumbrance when adventuring is.