i want to put longer blades on my heli. what should i leave as minimum clearance between tail rotor and main blades?

I would say 5 + mm min as long as , in know way they to would touch (tail to main).

I usually use a plumb bob off the tip of the blade that is in line with the tail boom. Then I also have the tail blades inline with the tail boom. IF the string off the tip of the blade does not come in contact with the tip of the tail blade or just touches the tip of the tail blade I don't think there will be problem. It is an easy way I have found to check clearance issues between main blades and tail blades.

David