I would like to know if the electrical panels come in one standard size for the various ways, that is to say, 18 way, 24 way, etc. What dimensions for each one of them?

You would be better served by reviewing some manufacturer's catalogs. These are the big three in North America:

http://www.geindustrial.com/catalog/buylog/00_BL_TC.pdf

Speedfax Catalog (http://www2.sea.siemens.com/Products/Controls/Catalogs/Speedfax.htm)

Schneider Electric US - Online Catalog (http://ecatalog.squared.com/)

There are standards based standard construction methods and dimensions for smaller panelboards, beyond that each manufacturer makes their own standards.