On Linux

dd is the command from Linux that i use. To create 1 GB file go with
dd if=/dev/zero of=testfile bs=1024 count=1024000

to create 100 MB try
dd if=/dev/zero of=testfile bs=1024 count=102400

and for 10 MB usedd if=/dev/zero of=testfile bs=1024 count=10240

You can find on other forum that they use /dev/urandom or /dev/random however this are load intensive for your server and take longer to make the file.

On Windows

For example, this command will create a 1GB file called 1gb.test on my desktop:

> fsutil file createnew c:\users\steve\desktop\1gb.test 1073741824

The key is to input the size of the file in bytes, so here are some common file sizes to save you from math:

1 MB = 1048576 bytes
100 MB = 104857600 bytes
1 GB = 1073741824 bytes
10 GB = 10737418240 bytes
100 GB =107374182400 bytes
1 TB = 1099511627776 bytes
10 TB =10995116277760 bytes