Artboards - Bleed and custom spacing when hitting the plus symbol

Bleed in grand format printing is a bit different. It is excess material that goes beyond the live area of the deisgn. Typically used for installation purposes. 6" of bleed will wrap around the billboard structure for instance. 

Live area = what you see once the print is installed. 
live area + bleed = (finished area) would be what you see if you open the box and view the print on the ground. 

Artboard bleed setting similar to Illustrator:
that need about 6" of bleed per side. I generally go to illustrator to create the artboards/bleed in there.

By @Jen5FFC

Bleeds are not only unavailable for artboards — they don't exist anywhere in Photoshop.

Both InDesign and Illustrator have bleed guides so you can draw your image off the page/artboard into an area that will be cut off when printing onto rolls of paper. Without the bleed, the paper might shift, leaving a very thin white strip along the edge of the paper. In the olden days before bleed guides, we created ruling guides in the pasteboard area. 

A standard bleed is 1/8" or 0p9 picas, not six inches. Nothing that is important should be in the bleed area, as anything in the bleed area gets cut off and thrown away when printed. Is that what you are trying to do? Or something else? The 6" confuses me, so there is something I don't understand in your post.

One limitation for Photoshop is that it does not have a pasteboard or area outside of the canvas to add bleed guides.

Jane

Agreed, artboards need more features!