Hi, you can use a Random Hexamer primer to create various forms (sizes) of dsDNA. However, achieving an exact size of ssDNA is not guaranteed due to the unknown primary sequence of the ssDNA. In this case, it is recommended to use a linker with a specific sequence at the initial part of the ssDNA, then use a primer (complementary to the linker sequence) and a polymerase to convert the ssDNA to dsDNA.