Well, the largest side is opposite the largest angle, but i'm assuming you're trying to find the length of the largest side.
Is your triangle a right triangle? If so, the long side would be the hypotenuse and you could find that with this equation...
a^2=b^2+c^2
Where a is the hypotenuse and b and c are the other two sides.
If it's an obtuse or scalene triangle then you could use the law of cosines...
a^2=b^2+c^2 - 2bc*cos(A)
Where a is the side you're looking for and b and c are the other sides and A is the angle opposite the side you're looking for.
That probably doesn't make much sense, but let me know if it helps! ^_^
