Hex map paper, Tabloid
Twelve-millimetre hexes across an 11 × 17 inch sheet — over a thousand cells, which is what a campaign map needs.
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Campaign scale wants area, not detail
An overland map is mostly about relationships: how far the mountains sit from the coast, which river valley connects two regions, how many days of travel lie between them. That calls for a great many hexes rather than large ones. At 12 mm on tabloid you get roughly 25 across and 45 down, which at six miles per hex is a continent rather than a county.
Six miles is the traditional hex
The six-mile hex has been the default for overland travel in tabletop games since the 1970s, for two reasons: it is roughly what a party covers in a day on foot through mixed terrain, and it divides neatly into seven sub-hexes of about two miles when you want to zoom in. Fix your scale before drawing terrain, because changing it afterwards means redrawing the map.
Printing and joining
Home printers rarely take tabloid. Print at 100% with tiling onto four Letter sheets and join them — and because hexes are geometric, a misalignment of a single millimetre at the seam is glaringly obvious. Tape from the back and butt the edges together rather than overlapping them. A print shop will run it in one piece for very little, which is usually the better answer.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100% — not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.