Hexagon paper, A3
A 10 mm hex grid on A3, which is the size a campaign map wants to be when several people are reading it.
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Maps get looked at by more than one person
A hex map on A4 is a map one person reads. A3 is what you put on the table. At 10 mm there are roughly 34 hexes across an A3 sheet and 45 down, which is over 1,500 cells: enough for a full region at any sensible scale, and still large enough per hex to write a terrain symbol or a number inside.
Numbering pays for itself immediately
A map that gets referenced during play needs coordinates, because describing locations by landmark stops working the moment two places look alike. The usual convention is a four-digit reference, column then row, so 0312 is column 3 row 12. Write the column numbers along the top and the rows down the side before you draw any terrain, since adding them later means working around whatever is already in the margins.
Printing A3
Home printers rarely exceed A4, but A3 is standard at print shops and costs very little. Taping two A4 sheets is a workable fallback for a map, where a seam matters less than it would on a technical drawing. What you should not do is scale an A3 sheet down to A4, since that takes 10 mm hexes to about 7 mm and quietly breaks any scale you had assigned to them.
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.