It’s simple, really. But you need to know where to look: Open the Finder window and hit “cmd & F” to bring up the search bar. Now select “Kind” and “Other…” and look for “File size”. By ticking the little box you can add it to the standard selection. What is left now, is to choose “is greater than,” enter a file size of your choice and there you have it! Just for choice: You could also go with this tool.