I have them too, Mine have grown to at least 5' the past few years now. I have had to trim them these past couple of years to keep them below the house windows, they seem to get alot thicker when i do. I started selective trimming with pruners, now i use the electric hedge cutters because it would take me forever doing each branch. I've pruned after the fall color is about done and the leaves are half dropped and i have pruned in the middle of the summer to keep them from growing over the sidewalk. I have both varities tall and lower mound type, so go ahead and prune and i think they will thicken up for you. I would cut off a third.

Myself, I would prune in the spring and then again in the summer or as needed. At a certain point I usually stop so the new growth doesn't get nipped by frost. Barberry actually respond well to shearing. I use hand shears just because I can get better cuts (easier to keep hand shears sharp for me).
1/3rd is a good rule to follow if you have to do some significant hacking. Who knows, maybe 1/2 might be needed in this case. I wouldn't suggest taking that much off unless I actually saw the plant.

If you have a plant that has been maintained then just take off all the scraggly stuff and perhaps go into the plant about 2 inches.

I am a staunch advocate of pruning with hand pruners, but I wouldn't waste my time on barberry.