I'm not a pro and I never fought amateur either but I sparred with both pros and amateurs...and I oddly enough had a better go of it fighting taller southpaws (go figure).

The keys to beating a taller/longer fighter are:
#1 Keep the head moving....not ALL fighters with long arms know how to use it ot their advantage. Slipping punches and shortening up the distance puts you in range and you may find your opponent off balance.

#2 Use angles...coming in straight forward gives the longer fighter the advantage of firing off straight punches keeping you at the end of them and therefore limiting your offense. Use headmovement and quick footwork to move inside and once you are there you can pivot to more effectively land hooks and uppercuts.

#3 Stay in your range but move out of their range...having shorter arms you have to use it to your advantage. When you aren't punching stay far away from your opponent so you can see what's coming or stay "in the pocket" and that takes the sting off of their punches. If you're in the pocket use your forearms and elbows, not as weapons but kind of like "the jaws of life" to open up some space for you to fire off punches.

What usually worked for me was to press the longer fighter against the ropes so that they couldn't use their long arms AND movement...it had to be one or the other. And if they got pinned straight up against the ropes then they were at my mercy so to speak.