A 40 gal
fuge on a 150 tank sounds fine for size.
Now to confuse you even more.
What function do you want the fuge to perform.
Ok now your thinking WTHeck is he rambling about. A fuge is a fuge right? Well not exactly. There are different types of fuges for different strokes. Now most folks when they setup a fuge their intent is to grow macro algae for filtration and possibly tang food but there are other reasons to do a fuge. For example your intent might be to provide a spot for pods
to grow to feed your tank or you might desire a place for your mantis to live and keep your snails and hermits safe or you might want to setup a mangrove fuge.
The most popular fuge would be the macro algae fuge and for this you will probably want a shallow sand bed (you could put your DSB
in the fuge and keep the tank bed shallow aswell). A few medium chunks of live rock to hold down your macro algae and then a your favorate macro algae. LIghting should be on a reverse cycle as compaired to your tank so as to provide some level of PH