Leeda Profil fly reel 5/6 weight

This is the reel that I am buying with my fly rod and I would like to know how much backing I could put on it?I fish a loch with brownies with the odd 2 lb trout.There is also sea trout in the lake and I want to be prepared in the event that I accidentely catch one.I am getting double taper fly line and for the second spool weight forward sinking line.I will mostly be using the floating line.I would like to know how much leader could go on it for trout fishing.I also fish stocked rainbows occasionally.

This is the reel: http://www.fishingmegastore.com/leeda-profil-fly-reel~4177.html

Tight Lines,
MadDog1999