How to make a file of the size you want ?
Let us assume you want to make a file of size 10000kb
Convert it to bytes -> 10000*1024 which comes out to be 10240000 bytes

Open calculator (win key+r -> type in calc )
Choose view mode to be Scientific one
Type in 10240000 and convert it to hex it comes out to be 9C4000
Convert this value to 8 digits by padding zeroes to its left -> so finally it comes out be 009C4000

Open Command Prompt (win key + r -> type in cmd )
Type DEBUG filename.dat
it'll show file not found error ,ignore it
Type RCX -> Enter -> then last four hexadecimal numbers i.e. 4000 ->Enter
Type RBX -> Enter -> then first four hexadecimal numbers i.e. 009C->Enter
Type W ->Enter
Type Q ->Enter

W stands for write and Q for quit

Now check the file you just made by dir command ->dir filename.dat

Using the same technique you can make files as big as you want

Here is an image to show how exactly everything can be done