Posted this on the microsoft answers forum but since I'm new here I'm not allowed post a link.
I'm a bit late coming to the party here but decided to post my solution here anyway.
I'm no expert but my work around involves editing the Onenote printer driver directly so that it takes a custom paper size. It mainly involved a bit of trial and error, some guesswork and in the end dumb luck but the solution is exactly what I was aiming for.
This should allow you to print Powerpoint slides of any size you specify to Onenote. I'm using Office 2010 and it works nicely. I take no responsibility if this doesn't work and you do this at your own risk. It's actually not that complicated though.
PART 1 - CREATE CUSTOM PRINT FORM
Step 1: Open Printer & Faxes from the Control Panel
Step 2: Go to File --> Server Properties and select the "Create New Form" checkbox.
Step 3: Enter the following measurements (or choose your own, I find these work well):
Units: Metric
Width: 8.8
Height: 11.7
Step 4: Name the form OneNote Powerpoint Custom (or whatever you feel like)
Step 5: Hit "Save Form" and exit by pressing OK
PART 2 - EDITING THE ONENOTE PRINTER DRIVER
Step 1. Locate the Generic Printer Description (GPD) file for Onenote. It is called sendtoonenote.gpd
On my system (Server 2003) it is located here: C:\WINDOWS\system32\spool\drivers\x64\3
Step 2: Make a copy of the above file and save it somewhere safe. This is just a backup so that you can restore a copy of the original file in case you make a mistake.
Step 3: Open the .gpd file in any text editor and locate the section "PageMediaSize - Paper Size".
Paste the following code anywhere after the line with *DefaultOption:
*Option: CUSTOMSIZE { *rcNameID: =USER_DEFINED_SIZE_DISPLAY *MinSize: PAIR(60, 100) *MaxSize: PAIR(32400, 32400) *MaxPrintableWidth: 32400 *MinLeftMargin: 0 *CenterPrintable?: FALSE *Command: CmdSelect { *Order: DOC_SETUP.2 *CallbackID: 3 } }
(Note: (if you're interested) This is just a standard piece of code I copied from the Windows Driver Kit (WinDKK) TTY Generic Text Only Printer Driver)
Step 4: (Optional) Change the default printer. On the line above where you just pasted in change the Default Option to CUSTOMSIZE (On my system the default was originally LETTER.
Step 5: Save and close the file
PART 3 - Printing
Open Powerpoint. Ctrl+P and select "Send to Onenote 2010". The first time you should select printer properties to see if your custom page form is selected.
Make sure you choose "Scale to Fit". The print preview should look no different than usual. There may be a slight "border" around the slide if you didn't use perfect settings in Part 1 - Step 3. If so, play around with those settings until you get it looking how you want.
Print away! Enjoy!
Note: You may not want your default page to be the custom one especially if you print to Onenote from other programs. If so simply skip Part 2 - Step 4 above.