The "pixels" are the abstract units used internally by Inkscape and the SVG graphics format. Formally, they are "user units" but the SVG standard makes "pixel" an option and that's what Inkscape calls them. By default in Inkscape, they map to real units at 90 to the inch.

As for scale, I'm not entirely sure what you are asking. Scale is the ratio of the distance in the map, to the real distance in the world being mapped. So if you wanted to have 100 miles map into say 7.5 inches to fit on a US Letter sized sheet of paper with a 0.5 inch margin, it would be 7.5 in : 100 miles = 1:844,800. Rounding up gives a nice even 1:1,000,000, and a 7.5 in map covers an extent of 118.371212 miles.

If you haven't already, I'd strongly recommend you run through the included tutorials under the help menu.