Where am I going wrong.

In assembley I can add 4 or more mating features but the brown box in the assembley tree

 stays the same size.How many mating faces does one need before the part is constrained.

it depends on the specific case, but 3 constraints is usually enough. also, if you are trying to constrain the current part to other parts that are not fully constraint themselves, you wont succeed.

post your problem on my id vedranchi2008@gmail.com with some pic, so i can clearly understand your problem and solve