Are LEGO Junior bricks bigger than regular LEGO?
I can't decide between Junior LEGO and Duplo for my son who is just over 3. I had thought the Junior sets might constitute a happy medium, but am now less sure.
LEGO Juniors sets use normal LEGO System bricks. In other words, they are exactly the same size as the "normal" LEGO bricks.
However, the Junior sets are specifically designed for younger children. They often have large single-use pieces (for example, for walls or frames), and are generally very easy to construct. They also limit the use of small or difficult to handle pieces as much as possible.
Depending on how advanced your son is, my guess is that Duplo is the most appropriate. If he no longer attempts to put toys in his mouth, and can focus/concentrate on a task for a decent amount of time, and has started developing fine motor skills, then Junior would probably be the most appropriate.
For reference, LEGO labels the Junior sets as appropriate for 4-7-year-olds, but I have met only a couple 4-year-olds that I would give the Junior sets to. Most 3-year-olds should probably be playing with Duplo.
Lego Juniors is fine for 3-year-olds who know not to put toys in their mouths. My son, when he was three, was playing with regular System Lego (of which Juniors is a subset) and it was okay. If it's important to you that your son have things like vehicles, or minifigs, and that he build the whole set himself, then Juniors is the way to go.
Otherwise, I see Lego as a longer-term investment: most kids enjoy playing with it from 3 to 13 or later. Stuff like the Juniors kits, or Duplo, gets to be too babyish after a couple years, but the regular stuff is still good forever. So if you just want your son to build, get a box of basic bricks (regular Lego bricks, not Duplo) and let him build. There are no instructions for him to worry about.
If he has any interest or aptitude he'll be ready for the kits that say they're for 5+ or 6+ in no time, and those kits will mesh well with the stuff you've already bought.