Run the following command:
fsutil file createnew <file> <size in bytes>
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