Being you're a boxer you don't want to just build muscle and size anyway.
What you need to do is lift for endurance, explosiveness, strength all with an emphasis on cardiovascular conditioning.
Pulling and Pushing exercises are ideal for your situation.
What you need to do is Low Weight/High Rep exercises.
For example instead of doing 4 sets of 10 of benchpress, you do 8 sets of 15 reps.
What it sounds like you want is to tone and define your frame which is exactly what I am doing now.
Here are some of the exercises I do for my training program
-Lunges
-Squats
-Dips
-Power Over Push-Ups
-Upright Rows
-Deadlifts
-Shrugs
-Burpees
-Compound Rows followed by Dumbell Curls
Find a program or routine that works for you. But remember your not trying to build mass just endurance and strength so do low weight high reps
Depending how old you are should determine your diet and supplement intake.
If this doesn;t help you then visit this link at MMA Forum because there are a lot more people with the same questions as you and more people are available there than they are on this forum.
There is a member there named Wukkadb and he should be able to help you out a lot more than I can.
http://www.mmaforum.com/training-nutrition/
