Brook Trout are far smaller. I think in the wild they cap out at roughly 10-12" though average size around 3-7 inches. (strictly experience based). Rainbows can get much larger. I would guess capping out at around 14-16" with an average size of 6-9 inches and can get quite fat. Agreed with the above mentioning that you need a lot of current, cold water and a bunch of oxegen, especially for the brookies because they are found native in fast cold mountain streams. I'm not sure where you can find info on keeping them, but I can suggest that it will take a lot of patience and setup and maybe include some fail rate.