If you want to do some download speed test, you can create a dummy file which have specific file size. To create an empty file with size you want type this command:
dd if=/dev/zero of=10mb-file.bin bs=1024k count=10
The command above will create a 10 MB sized file. If you want to create another file with different size, change the number after “count=”. Any number given after “count=” will be multiplied by 1024k or 1MB.