I would not recommend using HGH and testosterone to increase your arm size. Numerous dangerous, synthetic drugs can be used for this purpose, but they will not be legal to obtain without a prescription.
This being said, if you have 17′ inch arms naturally, then it is unlikely that you will gain 20 inches (~50 cm) in mere months by taking the previously mentioned substances. There are two reasons for this: 1) These drugs may cause significant damage to your body over an extended period if misused, so risks of bodily harm counterbalance the potential gains. 2) You do not have any underlying conditions or genetic mutations that would make such significant gains possible.
What should I do?
If you are dedicated to increasing the size of your arms, then there are several tried and tested methods that will give you notable gains. Weightlifting and other forms of resistance training can provide measurable results in toning and strengthening your muscles. In addition, you can positively impact the appearance of your arms by attending to diet and staying fit through cardiovascular exercise.
Please remember that going from 17′ inch (~43 cm) arms typically to 20′ (~50 cm) components would require a significant commitment because it would take years for such a change to occur naturally. You have no underlying genetic conditions or mutations that would make such significant gains possible naturally over short periods – but if you decide that you are willing to make the sacrifices necessary to get your desired results, then you can look forward to them! You cannot naturally get 20′ (~50 cm) arms if you have 17′ (~43 cm).