What is the diameter of the field of View of a microscopes LPO?

I am uming LPO is Low Power Objective. This is not enough information to answer, because you don't give a specific power for the objective, 4X maybe? Even if you know objective power, the field of view will differ from scope to scope. You can look at the manual, or the manufacturers website for this information. Or you can get a stage micrometer, see link, to measure the field.