Speaking of the big box...
As far as I am concerned, the "crazy cord," or "wild wire" extension cords are a 'best buy.'
They are 80-90ft. of 12/3 SJTOW wire, with molded-on ends. In simple english:
-SJ, while lighter than S, qualifies as heavy-duty insulation, and will last;
-T means shiny plastic on the outside, rather than dull rubber. I find it is more nick & cut resistant than rubber;
-O means oil & gasoline resistant; and,
-W means weather and sunlight resistant. (While tested to BEND at -40, it may not be very supple!)
Finally, the single molded connector will fit in a number of things that multiple-tap ends will not.
Typically, I will use one of these cords to serve several shorter (25') #14 cords.
Cords smaller than #14 are too prone to damage. I have not seen a need for #10, even over a 250' run. (If I was running a pipe threader, I might re-consider; load is everything in terms of voltage drop!)
I also have made extension cords using #12/2 zip cord (found with the Capri lights). I need this because not only do most of my tools have two-prong plugs, but many older homes have two-prong receptacles. I'd rather not use any adapters!
