Choose This Mac and Contents, then click the + button on the right.
In the first drop-down box, choose Other, then select Size.
In the next drop-down box, choose "is greater than", then input the minimum size of the files you want to find. Be sure to select KB, MB, or GB as appropriate.
Finder will show you all the files that match your criteria. You can then trash them or move them right from this window.