It depends on how long you plan on having the plant alive.
For a short week long veg and 8 week flower you can go the whole way in a gal pot/bag, but if you plan on vegging for any length of time you risk the plant becoming rootbound or potbound.
Any transplant into a larger pot sould be at least three times bigger than the original. If the plant is to survive it must be able to spread its roots toward rich soil.