Isometric graph paper, A5
The isometric lattice on A5, sized for a sketchbook or a pocket rather than a drawing board.
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Sketch where the object is
Isometric sketching is at its most useful in front of the thing you are drawing: a room you are measuring, a piece of furniture you are copying, a machine you are documenting. A4 and A3 belong on a desk. A5 goes in a bag and comes out on site, and a rough isometric sketch made in front of the object beats a careful one made from memory afterwards.
Five millimeters is about right at this size
Coarser and an A5 sheet holds too few intervals to be useful for anything with detail. Finer and the lattice starts to read as texture at reading distance. Five millimeters gives roughly 28 intervals across the short dimension, which is enough for an object of moderate complexity with room for a note beside it.
Measurements still hold
The property that makes isometric worth using does not depend on sheet size: all three axes are scaled identically, so a distance counted along any axis means the same thing. A sketch made on a train with the right number of intervals along each edge is dimensionally useful later, which is not true of a perspective sketch made the same way. Redraw it on A3 when it needs to be presentable.
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.