What is the usable space dimension , thickness of the pocket or usable space?
The pocket's interior is 5" wide x 6" tall. The depth of the pocket from the seams is about 1.5". However, the velcro flap on the front goes the entire length of the front flap , so you can probably stick a big item in there and safely let it expand to about 3" and still get a fairly secure closing of the flap on the velcro.
I added 2 of these to an Aeris EX Lite and they fit perfectly. I had to un-sew the nylon holding the waist pull to slid it on the belt ( it has slots for a 2 & 3" belt on the back), but a couple bucks and a few minutes at the upholstery shop fixed things back to where it was. I also shifted the waist band D-rings over to hold the XS pockets in place and sewed a little line around the D-ring to the waist belt to keep them in place. Great travel rig now with extra pockets for emergency stuff, including the two Aeris weight dump pockets just behind and next to the XS pockets.
I thing I noticed this morning, was that one of the 4 pockets didn't have the interior and exterior clips and hooks like the others, so I'll have to return it and do the sewing job all over, but not really a problem. Apparently a batch or earlier model went out without the interior d-ring and clip, which is good to have so nothing just floats away when you open the pocket.
These are well made and very good quality for the $$.
6 inches high, 6 inches wide, 4 inches "thick" or internal depth. You can get a LOT of stuff in here. I use it mostly for seashells, interesting items found underwater, etc. But I could put a lot of things in there - you'd have to really work hard at stuffing this thing.