A sieve is often chosen by its bowl diameter because that is the easiest dimension to see. The openings in the mesh can matter just as much. They help determine which particles pass through and which remain above the screen.
Name the separation you want
Are you removing large lumps from flour, separating seeds from a sauce or draining cooked vegetables? These tasks involve different particle sizes, textures and amounts of liquid. A tool that works quickly for one can clog or let unwanted material through in another.
A colander with relatively large holes should not be expected to behave like a fine-mesh sieve. Likewise, a very fine screen may slow a thick mixture so much that forcing it becomes tempting. Start with a tool suited to the recipe’s intended result.
Compare a small portion
If two sieves are available, try a modest amount of the same ingredient through each using a consistent, gentle method. Look at both sides: what collected below and what stayed behind? Speed alone does not tell you whether the separation was useful.
For an illustrative seed-containing sauce, one mesh may allow some seeds through while another retains them but also leaves more pulp. The preferred result depends on the texture you want. Pressing harder can change what passes, so record the technique as well as the tool.
Support the tool and respect its limits
Set the sieve securely over a suitable bowl. Follow its instructions about heat, load and contact with cookware. Do not assume a plastic handle or rim is safe resting against a hot pan. Avoid pushing with a sharp object that could damage the mesh.
Clean promptly according to the manufacturer’s guidance, checking the rim and screen for trapped food. A damaged mesh with loose wire is a reason to stop using the tool rather than work around the hole.
When buying a replacement, keep the useful details together: bowl size, mesh description, material, support hooks and cleaning requirements. The most helpful sieve is the one that gives the intended separation in your actual recipe, not simply the finest screen on the shelf.