I'm planning a 6' wide border and of course I need to reserve some room for a few Dahlias. How much room should be reserved for a giant or medium?
About the same as you'd reserve for a small- it's the blooms that are giant- not so much the plants. A miniature or BB can put out just as bushy a plant as an AA or A. The ones that are termed "Border Dahlias" take up less room because they are short- but they can still be bushy. We usually do 3' by 3' but they can be planted as close as 2' by 2'. Any more crowded than that & you're asking for trouble.
You can place stakes in the ground three feet apart. Plants two tubers per stake, one tuber on each side. BTW, I suggest 6-7 foot stakes made from steel (for concrete) reinforcement bar. Drive 18 inches into the ground. Use soft jute twine to tie them.