are just a growth (think goiter) that appears on redwoods from time to time. They are loaded with growth points. With the proper permits, folks harvest these commercially, usually seasonal.
They can be as small as a dime or larger than a truck. Many shapes and sizes make for good choosing when it comes to a display.
Limit the root zone and you can limit the growth. I have one that is many years old that is around 28" tall, bark on the "tree's" and moss all over the burl. requirements are....must stay moist on the bottom, thus a moss mat is best. They may be kept in any shallow tray, dish, bowl etc... Give them strong filtered light, as direct sun can burn the new sprouts easily.
With the size of the display tank you have it in Mass, it should get around 3' tall max.
I purchase all my burls from one man in Hiouchi, I trust him as harvesting honorably with good morals. Most are very cheap in price for the bang you get from them.
If anyone else would like to try growing a redwood...PM me with your addy and size of burl you would like. I can make a run over the border on weekends and fetch them up.