I am 80 years old and my prostate is large: shown in ultra sound as measuring 47 g. I am using antibiotic as prescribed by my doctor; I feel burning while passing the urine, please advise.

Did your doctor diagnose a Benign Prostate Hyperplasia (BPH), a prostatitis or another issue of the prostate? 

The most common reason for an enlarged prostate is BPH and in that case, Prostate 1 usually helps a lot.  Since your prostate is very large, it might not completely relieve your symptoms since the product is recommended for mild to moderate prostate enlargement. It will not make the excess cells disappear but will reduce some of the inflammation in the inner membranes of the prostate.  

You seem to be having a urinary tract infection.  If this is the case, make sure that you drink a minimum of 2 litres of water per day to help flush out the infection.  If the antibiotic does not seem to work, you could try A.Vogel Bladder Formula - Cystoforce for a maximum of 10 days.