Id say free weights, done in the right way, can be beneficial to any sport.

For boxing you want big, core exercises, that make your muscles work together (like they do in boxing)

Things like Deadlifts, Squats, Bench press and Military press are the things you should be looking at doing mainly. Along with other exercises like Dips and pullups, no muscle will go untouched.

DO NOT curl. Bicep isolation has been known to slow down punches.

Use a low rep/set range for strength. say 4 sets of 6 repetitions. Using a weight where your straining at the end of the set.

How often do you do cardio, whats your nutrition like? These things will come into play if you want to get bigger.