Question about random access file

Question

To test different cardreaders,I wrote a program to do the write/read data test.At first,copy a file of certain size form Harddisk to the cards,and then read the file form every card(s) and campare the data with the source file data.According to the capacity of each card,I should choose comparative file size from a ComboBox,such as 1MB,32MB,64MB,512MB,1GB,ect.Amd what's more,I can not know what the number my user will input to the ComboBox.So,my question is:If there is a way to let me read 1GB(or some other number>1MB) data from a file(size more small and more better.)? Thanks!