Probably the best homeowner guide to timber sizing was written by Ed Levin and published in Fine Homebuilding a number of years ago. That article, along with others along the same line, is now published in the Guild's Timber Frame Design Workbook. The workbook is the single best selling publication offered on this web site.
Engineers will do timber sizing for a fee. (We even do it for free if the cause is just.) See the answer under "Finding a licensed engineer in CA" for more detail on that subject.
