what is the avarage sizes for Brook trout in MA, up here in Idaho, they were introduced as a way of providing more actions for anglers, but they did more harm then good by conflicting with native fish for food, in some lakes and certain water ways we even introduce Tiger muskies in hoping to leviate some of the problems we have with them, and we have alot of them (brookies)in certain streams that are stunt, they only get so big, but theres a few that gets closed to the 10 inch mark.
, that stream has some large river trout for any controlled stream in Ma. I know that the Rainbow trout in thier are trophy size fish of 16"+ 
where can you catch a native brook trout in or around waltham??
You can find some small creeks and streams in Weston that have brookies, but they are a pain in the ass to get to and I have not gone to them in years. Try to google maps to identify and find access points. They are on the north side of town closer to 117.