Tough to say without an example to play with. If you need to just make the box wider, add this to your plot:
+ theme(legend.key.width = unit(5, "cm")
Personally, I also find that the width of the line in the legend can be too thin to see the color easily, so I also sometimes add something like this, which overrides the aesthetic mapping only in the legend:
+ guides(colour = guide_legend(override.aes = list(size = 1.2)))