If you are referring to the bit shaft diameters, those are standard dimensions. If doing heavy work, go with the 1/2" shaft.  

Hi

I'm not quite sure what you'd like to rabbet. If it is cove and bead strips I've found after some research a company delivering these bits with a 1/2" shaft.

www.toolstoday.com in the States has a flute and bead set-The item code of it is 54176. The price for was US$ 54.56 (without shipment) in February 2010, shipment to Euope was $ 11.50.

Tim,

Doesn't matter what the shank size is, either the 1/4" collet or the 1/2" collet will work. The important thing is to get the right rabbetting bit. The width will be spelled out for you, i.e., 3/8", 1/2", etc. The depth is set by your router. Some bits come as a set, that is one cutter and several different sizes of bearings you put on to change the width of the rabbet. Rockler has a good selection as does MLCS Woodworking. Hopes this helps.

George K

If you are building the dory, I found the required bit but it needed modification of the plate. I think it would have been easier to use a guide. I can give you more info if you want. 

The set with the bearings I highly recommend. MCLS stands behind there products as well. I had a problem with a keyhole bit. It broke almost immediately in some very hard wood.  Contacted MCLS, they replaced it with no question.