In the middlestages of a live MTT stack 25000 blinds 200/400/50 my plan was to raise 3.5bbs to try get HU from 4 off the button. All have similar stacks.
I noticed 2.5bbs raises getting 4 way. But one guy raised twice to 6bbs and get no callers.

Now I get 99 4 off button. I make it 1400. MP and btn and BB call, so 4 way.
KT2r. We all check.
brick turn we all check.
Q river. btn bluffs, called by two pair made on river.
I checkfold all the way.

Should I have made a bigger raise like to 6bbs with my 99 ?

Don't ask about bet sizing here. Most people would tell you to open 2.2xBB. Seems perfectly played on all streets. You are not always going to win with a top 5% hand.

The line you played is generally the more prudent way to go. You're too early in the orbit to make money by overbetting 99 and hoping that everyone folds so you can grab the blinds. Yeah, that move may work 60-70% of the time, but it doesn't net you much of a pot. By contrast, the times you get looked up will be both costly and unpleasant.

Stacks are deep enough to justify a smaller opening bet and set-mining with 99, which is what you did. You got to see a cheap river and avoided putting any more money in the pot when you were behind. Sorry you didn't hit a set in there.

Against that sort of loose passive table, you'll have better opportunities seeing cheap flops from the button and maybe the blinds, even with some marginal hands. Look to play on with well-concealed straights, 2-pair, trips, etc.