Q:

How to get my croissant flakier and bigger? Is it possible to get a hollow inside?

I am slightly new to the puff pastry process and have gotten the layers and and folding. I use a three fold, and the temperature remains at a cool degree so the butter does not melt or seep out of the dough during the process. 
I just can't seem to get them bigger and flakier. I am using 7in / 18cm triangles and I stretch them out and roll them into the classic croissant shape. Then I egg wash them, sprinkle a little sea salt on top, let rise for 40 minutes, bake at 400F/200C for 10 minutes, and then at 350F / 180C until golden brown on top. 
Any suggestion or any way to get the results?

A:

Croissant purists state 32 is the "perfect" number of layers a croissant should have.
In this link seeking the croissant perfection, you can find:

NOTE11, I had the misconception that the more folds, the more layers, the flakier it will be. Wrong. With too many folds, butter layers would be thinner and thinner, and it will be more likely for the butter to melt and leak. Even with perfect rolling, too may layers would mean smaller honeycomb "holes" in the crumb. With no sheeter and TX weather, I find 3 folds sufficient, any more it's risky.

You can get up to 27 layers if you make 3 3-folds. Doing one more folding would make 3^4=81, too many layers.
It's not compulsory, but you can get those 32 layers doing 1 normal-folding and 2 book-foldings:

(image source)

Another reason your flakes are not large enough is the flour used. 

Croissant dough usually has butter in it, and long fermentation time; so a strong flour (high W value) should be used.
In order the dough doesn't stretch back, and the layers of dough won't beak (letting butter from 2 layers get toghether), a value of p/l≈0.5.

That's something in common with pizza dough, as I wrote in this answer, so if you don't find flour specific for croissant, you can try with pizza one.

A:

If you are only proofing your shaped croissants for 40mins, that could be your problem.  Proofing croissants takes a lot longer than proofing bread.
you should be proofing the shaped croissants at 78degreesF/25~26degreesC for 2-2.5hours at HIGH humidity.  If you don't have a high humid environment, then put one coat of egg wash on right away before you proof (this keeps the butter from running).  The shaped croissants (after proper proofing) should be puffed up like "jabba the hut" and have a "jiggle" to them when you tap the baking sheet.

