I Want to Know the Size of the Sand Sieve am Using

If it's a purchased sieve, it will have the mesh size stamped on it.

Did you look?

You can send it to a lab for analysis, but you'll have to pay for the service, so, that's probably out.

The overall size can be simply measured.

The screen mesh can be counted as so many wires per centimeter or inch, using a magnifying glass, jeweler's loupe, or microscope as appropriate. Individual wires can be unwoven and measured with a micrometer, making it possible to compute the opening size. Or, wires of various gauges can be inserted into the mesh to see what fits.

could it be as easy as measuring the size of the particles that are able to pass though the sievee? , or do you need a factual measurement of the screen [wires?] spacing?

You can use machinist's wire gauges to measure the mesh openings. Do this at least 10 to 20 times to get a good mean average size reading.

If the mesh appears to be worn or there's evidence of heavily corrosion you may have to purchase a new well screen that has mesh openings one or two sizes smaller than your existing screen.

What type of mesh material is it?

Why don't you take it with you when you go to buy the new one, and ask them to match it? A sieve supplier should have no problem identifying the mesh size.

Codey