How are the waves like in Sumatra? Are they for “pro” surfers only?

NO!!! That’s a myth!
There are more than 50 waves at the Mentawai, more than 30 waves at the Telo islands, more than 20 at Nias, etc., etc., and of course there are waves of all kinds: hard-core, medium and easy waves, fast and slow waves, barrelling and not barrelling waves, etc. Therefore there are waves for all levels of surfing skills.
The average wave-size in Sumatra is 3-6 feet (1-2m). Occasionally the waves get bigger (8 feet/2,5m) but when that happens it’s only for 1 or 2 days, and even then usually there are alternative spots to surf smaller waves.  We know many spots for those surfers who don’t want to surf 8 feet barrels, so don’t worry, we will find the right waves for you.
Most waves break over coral reefs so a careful approach is needed particularly at low tide. But our surf-guide will not take you to surf on dangerous spots or in hard-core conditions.
Often our guests surf all the spots demanded on a trip.  Many of them tell us they’ve had the best surf of their lives, or the best barrels …or their first barrel ever!