The biggest factor is chain stay length. The most insignificant factor is derailleur arm length.

I subscribe to the big/big chain sizing method: Wrap your chain around the biggest front and rear sprockets, bypassing the derailleur. Figure out the shortest place you could rejoin the chain (there ware wide and narrow links). Add 1" of chain (2 links) to that. A 10 speed chain will come with it's own joining link or pin. Use that. That's the shortest chain that you can safely use with your gearing combination. In the big/big your derailleur arm will be pulled nearly horizontal.