Isometric graph paper, A3
The isometric lattice on A3, twice the area of A4, which is the size technical drawing is usually taught and examined on.
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A3 is the drawing board size
Outside North America, technical and engineering drawing is taught on A3, drawing boards are built for it, and coursework is submitted on it. A single object rendered isometrically with a couple of detail views does not fit comfortably on A4 once you allow for a title block and dimensions. If your work is going into a portfolio alongside printed drawings, matching the size is not cosmetic; it is what makes the folder usable.
Measurements stay true along all three axes
Isometric projection draws the three principal axes at 120 degrees to each other and scales all three identically. That is what separates it from perspective: you can measure a distance off an isometric sketch and it means something, whereas a perspective drawing looks more natural and tells you nothing dimensional. For design work that has to survive being redrawn by someone else, that property is the entire reason to use it.
Printing A3 when you only have an A4 printer
Most home printers stop at A4, but A3 is standard at print shops and in most offices and costs very little per sheet. Do not scale an A3 sheet down to A4 as a substitute, because that takes the grid to about 71% of the spacing you set and quietly destroys the measurement property that made you choose isometric. Print it at size or use the A4 sheet, which is ruled correctly for A4.
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.