Half-inch triangle graph paper
Equilateral triangles with half-inch sides, which is the working size for most patchwork planning.
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The size most triangle piecing actually uses
One-inch triangles are large enough to draw individual shapes clearly and too large to lay out a repeat. Quarter-inch is fine enough for dense pattern work and awkward to write inside. Half an inch sits where most planning happens: about seventeen triangles across a Letter sheet, big enough to number and color, small enough to see a whole block and its repeat.
Sixty degree piecing, not half-square triangles
Worth repeating because it catches people out. A half-square triangle comes from cutting a square across the diagonal and its legs sit on a square grid, so it belongs on half-inch graph paper. This sheet is for true equilateral triangles, the ones used in hexagon, diamond and tumbling block layouts, whose geometry does not fit a square grid at all.
The drawn line is the finished edge
As with all patchwork planning, the line you draw is where the seam sits, and the cutting line lies outside it by whatever allowance you use. Draw finished, cut with allowance. Sixty degree piecing also has more bias edges than square piecing does, since two of the three sides of every triangle are cut off grain, so handle the pieces gently and starch the fabric if it is prone to stretching.
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.