Pretty much anything with at least a 58-59cm(23in) top tube (measured from the centre of the steerer tube to the centre of the seat tube) will be rideable for you to get your foot in the door of riding. I myself at 6'5" spent 4 years on a 59cm top tube bike and even kept that bike as a permanent trainer bike once I had upgraded to a larger frame road bike. Bigger would be better, but something at least that size can get you out and about and maybe opens up more options for you to look at. Also, rather than just looking at pure road bikes, take a look at city and hybrid bikes. You can generally find them well within your price range and barely used. You could even stretch the budget a little and buy one brand new from the likes of Giant/Trek