I would refer to the following Article: " Molecular weights of Plant Viral Coat Proteins by Polyacrylamide  gel Electrophoresise", Virology, Vol. 47, Issue 3, 817-822. If you are familiar with SDS-PAGE, you should identify the specific % Acrylamide gel you need for the Molecular weight you are looking for and then identify a molecular weight marker that will be best for that particular gel. Using the Rf Values of each Molecular weight marker protein, you should then be able to identify the molecular wieght of the your viral coat protein.

Unless there is something unusual about your protein, you can use standard SDS-PAGE methodology. To determine molecular weight, using a single acrylamide concentration gel (not a gradient gel), and a set of commercial molecular weight markers run in the lane next to your protein. Plot a graph of Rf versus log molecular weight of the standards, and use the Rf of your protein and the standard curve to measure the molecular weight of the monomer. (Rf is the ratio of the distance from the top of the separating gel to the band divided by the distance from the top of the separating gel to the dye front.).

If you are interested in knowing the molecular weight of the oligomer, then you should not use SDS-PAGE. There is a procedure to determine the molecular weight by non-denaturing PAGE, which involves running the protein on gels with varying acrylamide concentrations. You could also use analytical ultracentrifugation, sucrose gradient ultracentrifugation with size standards, and size-exclusion chromatography coupled with multiangle light scattering, all of which require specialized equipment.

Thank for your useful answers

For a more specific estimation of molecular weight MS would be more helpful.