depending on what size you get him at, i would expect they should reach no less than about 12"+ in a year...
If he's the only fish in the 20 should be fine for a few monthes.. if not.. he's likley to jump. ime they are extreamely skittish and reclusive and need space and tlc to settle into your tank. They can grow ime as fast as a RTC or silver arrowana 1"+ a month or more and reach 12"+easily by their first year if not before that. your 100 gallon will last approx 2yrs before it will be outgrown.