1) A monomer isn't always a monosaccharide, but all monosaccharides are monomers.
2) A polymer can be a chain of monomers, or repeating units. The basic principal behind polymers is that it is composed of many repeating units.
3) Taking the definition from (2), my opinion would be a dissacharide would only be a polymer if it were constructed to two of the same monomer, e.g. maltose (2 glucose subunits). So TECHNICALLY, it is a polymer, since its composed of two identical ("repeating") subunits.
In a disaccharide composed of two different monosaccharides (e.g. sucrose, from 1 glucose and 1 fructose subunit), there is no pattern or repetition, so I don't think it would be a polymer, and therefore not a polysaccharide.
4) Typically, polysaccharides are in the order or 100s or 1000s of linked monosaccharides.