What size dowels to use for joints?
The best work I know of on the topic was written by Tage Frid (see http://amazon.com).
The approximate rule-of-thumb, iirc, is to use a dowel no more than 3/8 the thickness of the wood to be joined (and not less than 1/4 the thickness).
Expansion
For example, if you are joining 5/4 boards (finished thickness 1&quot;), the dowels should be between 1/4&quot; and 3/8&quot; in diameter.
Likewise, if joining finished thickness 3/4&quot; boards, use between 3/16&quot; and 9/32&quot; diameter dowels.
This presumes, of course, you're joining the boards on the narrow dimension (e.g. making a door frame).